Liquid crystal display device and polymer for alignment film materials

ABSTRACT

A liquid crystal display device suppresses generation of image sticking in AC mode. A liquid crystal display device comprises an alignment film arranged on at least one of a pair of substrates. The alignment film comprises a polymer containing a first constitutional unit and a second constitutional unit. The first constitutional unit controls alignment of the liquid crystal molecules by photoirradiation. The second constitutional unit controls alignment of the liquid crystal molecules regardless of photoirradiation.

This application is a continuation of U.S. patent application Ser. No.12/593,121, filed 25 Sep. 2009, which is the U.S. national phase ofInternational Application No. PCT/JP2008/053315, filed 26 Feb. 2008,which designated the U.S. and claims priority to Japanese PatentApplication No. 2007-080289, filed 26 Mar. 2007, the entire contents ofeach of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device and apolymer for alignment film materials. More particularly, the presentinvention relates to a liquid crystal display device with wide viewingangle characteristics, preferably used in planar displays that are usedby many people, such as a PDA, a PC, a WP, amusement equipment, ateaching machine, and a TV device, or in a display board, a displaywindow, a display door, a display wall, etc., each utilizing a shuttereffect of liquid crystal. Further, the present invention relates to apolymer for alignment film materials.

BACKGROUND ART

A liquid crystal display device is now being widely used attributed toits characteristics such as slim profile, light weight, and lowelectrical power consumption. The liquid crystal display device includesa pair of substrates and a liquid crystal layer interposed therebetween. Further, the liquid crystal device provides display bycontrolling an alignment direction of liquid crystal molecules containedin the liquid crystal layer by appropriately applying a voltage toelectrodes arranged on liquid crystal layer side-surfaces of thesubstrates. The liquid crystal display device usually includes analignment film for controlling the alignment direction of the liquidcrystal molecules, and the alignment film is arranged on the liquidcrystal layer side-surface of the substrate.

As a material for such an alignment film constituting the liquid crystaldisplay device, resins such as polyamic acids, polyimides, polyamides,and polyesters are conventionally used. Among them, polyimides have beenmuch used for liquid crystal display devices attributed to its excellentphysical properties such as heat resistance, affinity with liquidcrystals, and mechanical strength compared with other organic resins.

The alignment film is typically subjected to an alignment treatment, andthereby the film can provide liquid crystal molecules that arepositioned on the alignment film surface with specific pretilt angles. Arubbing method, a photo-alignment method, and the like, are mentioned asa method for the alignment treatment. According to the rubbing method,the alignment film surface is provided with the alignment treatment bybeing rubbed with a cloth wound on a roller. According to thephoto-alignment method, a photo-alignment film is used as a material forthe alignment film, and the photo-alignment film is irradiated with(exposed to) light such as ultra violet light, and thereby the alignmentfilm is provided with an alignment regulating force and/or an alignmentregulating direction of the alignment film is changed.

However, according to a liquid crystal display device including aconventional alignment film, image sticking might be generated on ascreen when an image is displayed for a prolonged period. So there isroom for improvement in that the image sticking is suppressed even whenan image is displayed for a prolonged period.

In order to solve such a problem, for example, Patent Document 1discloses a liquid crystal aligning agent composition containing atetrafunctional silicon composition like tetrachalcoxylan, atrifunctional composition like trialcoxylan, and a product of reactionwith 0.8 to 3.0 mol water for a 1 mol functional group like an alkoxygroup, and a glycol ether solvent, as a technology of providing a liquidcrystal aligning agent capable of forming a liquid crystal alignmentfilm which prevents display defects, has an excellent afterimagecharacteristic even after long-time driving, does not decrease thecapability of aligning liquid crystal, and has a small decrease involtage holding rate against light and heat.

In addition, for example, Patent Document 2 discloses a liquid crystalalignment material including a polyamic acid having a structure derivedfrom a monoamine compound or its imidized polymer, as a technology ofproviding a liquid crystal alignment material which can develop goodcoating film formability and liquid crystal alignment characteristicsand can form a liquid crystal alignment layer capable of deletingafterimages in a short time after the stop of the application of avoltage in liquid crystal display elements.

In addition, for example, Patent Document 3 discloses a vertical liquidcrystal aligning agent including 100 weight parts of a polymer with anamic acid repeating unit and/or an imide repeating unit and at least 5weight parts of a compound having at least two epoxy groups in amolecule, as a technology of providing a liquid crystal aligning agentwhich provides a vertical liquid crystal alignment film excellent inimage sticking characteristics and reliability even when the film isused with a reflection electrode.

For example, Non-patent Document 1, which is a document on anphoto-alignment film, discloses that the smaller an electricalresistivity of a photo-alignment film is, the shorter an image stickingtime is.

Further, for example, Non-patent Document 2, which is a document ondevelopment of a material for alignment films, discloses that areduction in residual DC in a liquid crystal cell with a verticalelectrical field leads to suppression of the image sticking.

In an AC driving liquid crystal display device, a residual DC isgenerated generally due to a difference in an offset voltage betweenelectrodes formed on substrates facing each other.

In addition, for example, Patent Document 4 discloses polyimidescontaining a side chain group having a structure that can be derivedfrom 3-arylacrylic acid, as photoreactive polymers that can produce astable and high-resolution alignment pattern that show a defined pretiltangle and at the same time has a sufficient high resistance value(holding ratio) in a liquid crystal medium adjacent to the pattern whenthe pattern is irradiated with polarized light.

For example, Patent Document 5 discloses polyimides, incorporatingcinnamic acid group derivatives in such a way that the cinnamic acidgroups are linked to the polyimide main chain via a carboxylic group bymeans of a flexible spacer, as photoreactive polymers that can produce astable and high-resolution alignment pattern that show a very large tiltangle and at the same time has a sufficient high holding ratio in aliquid crystal medium adjacent to the pattern when the pattern isirradiated with polarized light.

PATENT DOCUMENT 1

-   Japanese Kokai Publication No. 2005-250244

PATENT DOCUMENT 2

-   Japanese Kokai Publication No. 2006-52317

PATENT DOCUMENT 3

-   Japanese Kokai Publication No. 2006-10896

PATENT DOCUMENT 4

-   Japanese Kohyo Publication No. 2001-517719

PATENT DOCUMENTS 5

-   Japanese Kohyo Publication No. 2003-520878

NON-PATENT DOCUMENT 1

-   Masaki HASEGAWA, “HIKARI HAIKOU-seis an prosesu no kanten kara mita    haikoushori,” liquid crystal, The Japanese Liquid Crystal Society,    Jan. 25, 1999, vol. 3, No. 1, p. 3-16

NON-PATENT DOCUMENT 2

-   Kiyoshi SAWAHATA, “LCD you haikoumaku no zairyou kaihatsu doukou,”    liquid crystal, The Japanese Liquid Crystal Society, Oct. 25, 2004,    vol. 8, No. 4, p. 216-224

DISCLOSURE OF INVENTION

An image sticking phenomenon that is generated in residual DC(direct-current) mode has been a commonly known as the image stickingphenomenon in liquid crystal display devices. So a material for reducingthe residual DC has been researched and developed as a measure for theimage sticking on an alignment film material surface. The image stickingin DC mode can be dealt with by a conventional technology, for example,using a material (molecule) that less accumulates charges.

However, in the photo-alignment technology, which is now being employedinstead of the rubbing method, the mechanism for generation of the imagesticking is not determined yet, and so a solution for it is not proposedyet. For example, in the vertical photo-alignment films disclosed inPatent Documents 5 and 6, in addition to strong image sticking inresidual DC mode, image sticking (in AC (alternative current) mode)caused by a change in pretilt angle by AC voltage application isgenerated. So the image sticking in both modes needs to besimultaneously solved.

A homopolymer or copolymer of only the photo-alignment film material (s)is not enough to solve particularly the image sticking in AC mode. Inaddition, according to an alignment film in which a conventionalphotofunctional group-containing vertical photo-alignment film and aconventional vertical alignment film are blended, uniformity ofalignment is significantly reduced because a density of thephotofunctional group-containing vertical photo-alignment film isreduced, and further, the alignment film can not provide liquid crystalswith pretilt angles and so the pretilt angle hardly changes from 90°.The conventional vertical alignment film contains a vertical alignmentfunctional group and has characteristics of aligning liquid crystalmolecules in a direction substantially vertical to the alignment filmsurface without being provided with an alignment treatment such asrubbing and photoirradiation. In the alignment film in which thevertical photo-alignment film and the vertical alignment film areblended, the image sticking in AC mode is probably caused due to apolymer that is contained in a photo-alignment film material. So such afilm is not enough to suppress the image sticking in AC mode.

The image sticking cause by AC voltage application is particularlystrongly observed if a photo-alignment film (homopolymer) containing aphotofunctional group that undergoes a photoreaction (e.g., aphotocrosslinking reaction (including a photodimerization reaction), aphotoisomerization reaction, a photodecomposition reaction) therebyproviding a liquid crystal molecule with a pretilt angle, is used.

Liquid crystal display devices where a liquid crystal alignmenttreatment is provided in one direction in the substrate plan, like TN(twisted nematic), ECB (electrically controlled birefringence), and VATN(vertical alignment twisted nematic) liquid crystal display devices,etc., show display characteristics depending on viewing angle. So thedirection where the image sticking phenomenon might be observed is thefront direction and some directions different depending on the viewingangle characteristics of the devices themselves. In a liquid crystal TV,a large-screen advertising display, and the like, liquid crystals arealigned in different directions in view of viewing angle compensationunder white display state. Thus, in the multi-domain mode where theviewing angle is compensated, the image sticking phenomenon is uniformlyobserved in every azimuth, and so suppression of the image stickingphenomenon is essential. In the present description, the VATN mode maybe a so-called RTN (reverse twist nematic: vertical alignment in TNmode). Further, the ECB mode may be VAECB mode where liquid crystals arevertically aligned during non-voltage application and horizontallyaligned during voltage application, or may be mode where liquid crystalsare vertically aligned during voltage application and horizontallyaligned during non-voltage application,

The present invention has been made in view of the above-mentioned stateof the art. The present invention has an object to provide a liquidcrystal display device capable of suppressing generation of the imagesticking in AC mode and a polymer for alignment film materials.

The present inventors made various investigations on a liquid crystaldisplay device capable of suppressing generation of the image stickingin AC mode (herein after, also referred to as “AC image sticking”) and apolymer for alignment film materials contained in the alignment film.The inventors noted a mechanism for generation of the AC image sticking.Then, the inventors found that the following two are mentioned as themechanism for generation of the AC image sticking. One is that a sidechain part of the alignment film deforms by a stress caused by elasticdeformation of a liquid crystal molecule (side-chain deformationmemory). The other is that a liquid crystal molecule is adsorbed to afunctional group whose main chain has a high polarizability by ACapplication (liquid crystal adsorption).

The reasons why the AC image sticking is generated in the conventionalphoto-alignment film are mentioned in more detail below with referenceto FIGS. 22 and 23. FIG. 22 is a cross-sectional view schematicallyshowing a vicinity of a conventional photo-alignment film surface andexplaining a mechanism for generation of AC image sticking due toside-chain deformation. FIG. 22( a) shows an initial state. FIG. 22( b)shows a state when an electrical field is applied to a liquid crystallayer. FIG. 22( c) shows a state when the application of the electricalfield to the liquid crystal layer is stopped. Further, FIG. 23 is across-sectional view schematically showing a vicinity of a conventionalphoto-alignment film surface and explaining a mechanism for generationof AC image sticking due to liquid crystal adsorption. FIG. 23( a) showsan initial state. FIG. 23( b) shows a state when an electrical field isapplied to a liquid crystal layer. FIG. 23( c) shows a state when theapplication of the electrical field to the liquid crystal layer isstopped.

The mechanism for generation of the AC image sticking due to theside-chain deformation is mentioned below. As shown in FIG. 22( a), inthe initial state, liquid crystal molecules 111 contained in a liquidcrystal layer 120 and side chains 131 of a photo-alignment film 130interact with each other, and the molecules 111 are pre-tilted. Then, asshown in FIG. 22( b), when an electrical field is applied to the liquidcrystal layer 120, the molecules 111 are bend-deformed and an elasticenergy due to the bending deformation is generated, and then, the sidechains 131 are inclined in accordance with the molecules 111 in order todecrease the elastic energy. Then, as shown in FIG. 22( c), when theelectrical field is stopped, the side chains 131 exhibit its restoringforce. However, the interface of the photo-alignment film 130 iscrystallized compared to a bulk of the liquid crystal layer 120. So areleasing time it takes for the side chains 131 to return to itsoriginal structure is long. During the releasing time, the pretilt angleof the liquid crystal layer 120 is also changed, resulting in generationof the image sticking.

The mechanism for generation of the AC image sticking due to the liquidcrystal adsorption is mentioned below. That is, when light such as ultraviolet light is radiated to the photo-alignment film 130, a space insize equivalent to the liquid crystal molecule 111 is generated betweenmolecules (main chains 125) of the photo-alignment film 130. In theinitial state, as shown in FIG. 23( a), a part where the liquid crystalmolecule 111 is adsorbed to the main chain 125, that is, an adsorbingliquid crystal, exists. Then, as shown in FIG. 23( b), when anelectrical field is applied to the liquid crystal layer 120, themolecules 111 are bend-deformed and an elastic energy due to the bendingdeformation is generated, and then, the molecules 111 that are near thephoto-alignment film 130 pass through the side chains 131 and alignedwith the absorbing liquid crystal. As shown in FIG. 23( c), when theapplication of the electrical field to the liquid crystal layer 120 isstopped, the side chain 131 exhibits its restoring force, but theinterface of the photo-alignment film 130 has a larger adsorbingproperty compared with a bulk of the liquid crystal layer 120. So areleasing time it takes the liquid crystal molecules 111 near thephoto-alignment film 130 to show an original tilt angle again is long.During the releasing time, the pretilt angle of the liquid crystal layer120 is also changed, resulting in generation of the image sticking.

The inventors further made investigations, and then found thefollowings. If a polymer contained in an alignment film materialincludes the following constitutional units. One constitutional unit issensitive to light and is likely to be easily provided with an alignmenttreatment by photoirradiation, but easily undergo the side-chaindeformation or cause the liquid crystal adsorption. The otherconstitutional unit can control alignment regardless ofphotoirradiation. That is, a film formed from an alignment film materialincluding a polymer containing a constitutional unit exhibiting aproperty of controlling alignment of the liquid crystal molecules byphotoirradiation and a constitutional unit exhibiting the property ofcontrolling alignment of the liquid crystal molecules regardless ofphotoirradiation is provided with an alignment treatment byphotoirradiation, and thereby the AC image sticking can be suppressed.As a result, the above-mentioned problems have been admirably solved,leading to completion of the present invention.

That is, the present invention is a liquid crystal display deviceincluding:

a pair of substrates;

a liquid crystal layer containing liquid crystal molecules; and

an alignment film,

the liquid crystal layer being interposed between the pair ofsubstrates,

the alignment film being arranged on a liquid crystal layer side-surfaceof at least one of the pair of substrates,

wherein the alignment film is obtain able by providing a film with analignment treatment by photoirradiation,

the film being formed from an alignment film material,

the alignment film material including a polymer containing a firstconstitutional unit and a second constitutional unit,

the first constitutional unit exhibiting a property of controllingalignment of the liquid crystal molecules by photoirradiation,

the second constitutional unit exhibiting the property of controllingalignment of the liquid crystal molecules regardless of photoirradiation(herein after, also referred to as a “first liquid crystal displaydevice of the present invention”).

The first liquid crystal display device of the present invention isdescribed in more detail below.

According to the first liquid crystal display device of the presentinvention, a liquid crystal layer containing liquid crystal molecules isinterposed between a pair of substrates, and an alignment film isarranged on a liquid crystal layer-side surface of at least one of thepair of substrates.

The configuration of the first liquid crystal display device of thepresent invention is not especially limited as long as the first liquidcrystal display device essentially includes such standard components acommon liquid crystal display device has. The first liquid crystaldisplay device may or may not include other components.

The first liquid crystal display device of the present invention may bea passive matrix liquid crystal display device, but preferably an activematrix liquid crystal display device. Thus, it is preferable that thefirst liquid crystal display device includes pixels arranged in a matrixpattern,

the pixels including a pixel electrode and a common electrode,

the pixel electrode being arranged in a matrix pattern on a liquidcrystal layer side-surface of one of the pair of substrates, and

the common electrode being arranged on a liquid crystal layerside-surface of the other substrate.

It is preferable that the alignment film is arranged on liquid crystallayer side-surfaces of the both substrates in order to improve displayqualities and responsiveness of the first liquid crystal display device.

In order to more suppress the AC image sticking, it is preferable thatthe first liquid crystal display device includes, on liquid crystallayer-side surfaces of the both substrates, an alignment film obtainable by providing a film with an alignment treatment byphotoirradiation, the film being formed from an alignment film material,the alignment film material including a polymer containing a firstconstitutional unit and a second constitutional unit, the firstconstitutional unit exhibiting a property of controlling alignment ofthe liquid crystal molecules by photoirradiation, the secondconstitutional unit exhibiting the property of controlling alignment ofthe liquid crystal molecules regardless of photoirradiation.

The above-mentioned alignment film is obtain able by providing a filmwith an alignment treatment by photoirradiation, the film being formedfrom an alignment film material including a polymer containing a firstconstitutional unit and a second constitutional unit. The firstconstitutional unit exhibits the property of controlling alignment ofthe liquid crystal molecules by photoirradiation. The secondconstitutional unit exhibits the property of controlling alignment ofthe liquid crystal molecules regardless of photoirradiation. Accordingto this, the AC image sticking can be suppressed even if the alignmentfilm is provided with an alignment treatment by photoirradiation, andthereby a liquid crystal display device having excellent displayqualities can be provided, and advantages in terms of productionprocesses of the photo-alignment method can be taken. In addition, acoating property of the alignment film material can be improved. Forexample, the advantages of the photo-alignment method are as follows:the alignment film can be subjected to the alignment treatment in acontact-less manner, and so soils, dusts, and the like, which aregenerated by the alignment treatment, can be reduced; generation ofdisplay defects (such as rubbing line), which might be caused by amechanical alignment treatment such as a rubbing method, can besuppressed; and the alignment film is exposed through a photomask wheretransmissive parts are formed in a desired pattern, and thereby eachpixel can be easily divided into plural domains with a desired design(planar shape).

In the polymer, how the constitutional units are distributed is notespecially limited. The polymer may be any of an alternating copolymer,a block copolymer, a random copolymer, and a graft copolymer. Themolecular weight of the polymer is not especially limited, but it ispreferable the polymer has a molecular weight suitable for use as thealignment film, similarly to a polymer contained in a conventionalalignment film material. The proportion of each constitutional unit inthe polymer is not especially limited, but a preferable ratio (% byweight) between the two constitutional units is mentioned below.

The alignment film is provided with an alignment treatment byphotoirradiation (preferably, ultra violet light-irradiation). So it ispreferable that the alignment film is sensitive to light, particularlyUV light, and more specifically, it is preferable that the alignmentfilm reacts to light, particularly UV light at a smaller exposure energyin a short time. In order to shorten a tact time in the productionprocess, the alignment film is preferably photoirradiated at an exposureenergy of 100 mJ/cm² or less, and more preferably at an exposure energyof 50 mJ/cm² or less. If the alignment film is provided with analignment treatment by compartmentalizing each pixel region into someregions and separately exposing the regions through a light-shieldingmask (photomask) and the like, it is preferable that the alignment filmis photoirradiated at an exposure energy of 20 mJ/cm² or less.

Preferable embodiments of the first liquid crystal display device of thepresent invention are mentioned in more detail below.

As means for exhibiting the property of controlling alignment of theliquid crystal molecules by photoirradiation in the first constitutionalunit, a photofunctional group is preferable, and particularly aphotofunctional group contained in a side chain of the firstconstitutional unit is preferable. As a result, the first liquid crystaldisplay device can be more easily provided, and the AC image stickingcan be more effectively reduced. Thus, it is preferable that the firstconstitutional unit contains a photofunctional group, and it is morepreferable that the first constitutional unit has a side chaincontaining a photofunctional group.

In the present description, the photofunctional group is not especiallylimited as long as it is a functional group capable of exhibiting theproperty of controlling alignment of the liquid crystal molecules byphotoirradiation. The photofunctional group is preferably a group thatcan undergo at least one of a crosslinking reaction (including adimerization), a decomposition reaction, an isomerization reaction, anda photorealignment reaction, more preferably at least one of acrosslinking reaction (including a dimerization), a isomerizationreaction, and a photorealignment reaction, by light, preferably UVlight, and more preferably polarized UV light.

As means for exhibiting the property of controlling alignment of theliquid crystal molecules regardless of photoirradiation in the secondconstitutional unit, an alignment functional group is preferable, and analignment functional group that is contained in a side chain of thesecond constitutional unit is particularly preferable. As a result, thefirst liquid crystal display device can be more easily provided, and theAC image sticking can be more effectively reduced. Thus, it ispreferable that the second constitutional unit contains an alignmentfunctional group, and it is more preferable that the secondconstitutional unit has a side chain containing an alignment functionalgroup.

The alignment functional group is not especially limited as long as itis a functional group capable of exhibiting the property of controllingalignment of the liquid crystal molecules regardless ofphotoirradiation, and publicly known alignment functional groups such asa vertical alignment functional group and a horizontal alignmentfunctional group may be used. The vertical alignment functional group isnot especially limited as long as it is a functional group capable ofexhibiting a property of vertically aligning liquid crystal molecules,but preferably a functional group capable of exhibiting such a propertyby rubbing or without any treatment, more preferably without anytreatment, i.e., without the alignment treatment. The horizontalalignment functional group is not especially limited as long as it is afunctional group capable of exhibiting a property of horizontallyaligning liquid crystal molecules, but preferably a group capable ofexhibiting such a property by rubbing or without any treatment.

Thus, the first liquid crystal display device may be a liquid crystaldisplay device including: a pair of substrates; a liquid crystal layercontaining liquid crystal molecules; and an alignment film, the liquidcrystal layer being interposed between the pair of substrates, thealignment film being arranged on a liquid crystal layer side-surface ofat least one of the pair of substrates, wherein the alignment film isobtain able by providing a film with an alignment treatment byphotoirradiation, the film being formed from an alignment film material,the alignment film material including a polymer containing the followingtwo constitutional units: one containing a photofunctional group andcontrolling an alignment direction of liquid crystal molecules that arepositioned on the alignment film surface by photoirradiation to thealignment film; the other containing an alignment functional group andcontrolling an alignment direction of liquid crystal molecules that arepositioned on the alignment film surface regardless of photoirradiationto the alignment film.

It is preferable that the first and second constitutional units alignthe liquid crystal molecules in the same direction. As a result, thefirst liquid crystal display device can be effectively driven in asingle liquid crystal mode such as VATN, TN, ECB, IPS (in-planeswitching) modes. The same direction is not necessarily strictly thesame direction, and may be almost the same direction as long as thedevice can be driven in a single liquid crystal mode.

From the same viewpoint, it is preferable that the alignment filmuniformly controls the liquid crystal molecules in a plane of thealignment film. In the present description, the term “uniformly” doesnot necessarily mean that the liquid crystal molecules are alignedstrictly uniformly as long as a single liquid crystal mode can beachieved.

In order to effectively drive the first liquid crystal display device inVA mode such as VATN mode, it is preferable that the alignment film is avertical alignment film that aligns the liquid crystal moleculesvertically. In the present description, the term “vertically” does notnecessarily mean that the liquid crystal molecules are aligned strictlyvertically to the alignment film surface, and may be aligned verticallyto the alignment film surface to such an extent that VA mode such asVATN mode can be achieved.

More specifically, in order to effectively drive the first liquidcrystal display device in VA mode such as VATN mode, it is preferablethat the alignment film aligns the liquid crystal molecules in such away that an average pretilt angle of the liquid crystal layer is 87° to89.5°, more preferably 87.5° to 88.5°. As a result, the liquid crystaldisplay device in VATN mode excellent in viewing angle characteristics,responsiveness, and light transmittance, can be provided. Morespecifically, in order to suppress adverse effects on contrast in VATNmode (suppress increase in luminance under black display state), it ispreferable that the alignment film aligns the liquid crystal moleculesin such a way that an average pretilt angle of the liquid crystal layeris 87° or more, and more preferably 87.5° or more. In order to suppressan afterimage that is generated when a display face is pressed and toadjust an extinction position within a plus or minus 5° under theconditions of: polarization plates that are arranged in Cross-Nicolstate are rotated 45°; and a voltage applied to the liquid crystal layerof 7.5V, it is preferable that the alignment film aligns the liquidcrystal molecules in such a way that an average pretilt angle of theliquid crystal layer is 89.5° or less, and more preferably 88.5° orless.

In order to effectively drive the first liquid crystal display device inVA mode such as VATN mode, it is preferable that the secondconstitutional unit has a side chain containing a vertical alignmentfunctional group. As a result, the liquid crystal display device in VAmode such as VATN mode can be easily provided.

In the present description, the average pretilt angle of the liquidcrystal layer is an angle made by a substrate surface and a direction(polar angle direction) of an average profile (director) of liquidcrystal molecules in the thickness direction of the liquid crystal layerunder no voltage application between the substrates. An apparatus formeasuring the average pretilt angle of the liquid crystal layer is notespecially limited, and a commercially available tilt angle-measuringapparatus (product of SHINTEC, Inc., trade name: OPTIPRO) may bementioned, for example. According to this apparatus, a substrate surfaceis defined as 0° and the direction vertical to this substrate surface isdefined as 90°, and the average profile of liquid crystal molecules inthe thickness direction of the liquid crystal layer is measured as apretilt angle. So such an apparatus is preferably used to measure theaverage pretilt angle. It is considered that the average pretilt angleof the liquid crystal layer depends on a profile of liquid crystalmolecules near an alignment film (on an interface between the liquidcrystal layer and the alignment film), and the liquid crystal moleculesthat are positioned on the interface induce elastic deformation ofliquid crystal molecules in the bulk (middle) of the liquid crystallayer. In addition, it is considered that the profile of the liquidcrystal molecules is different between the vicinity of the alignmentfilm (interface) and the bulk (middle) of the liquid crystal layer, andto be exact, the directions of profiles (polar angle directions) of theliquid crystal molecules are also different between the two.

The following embodiments are preferable in order to effectively drivethe first liquid crystal display device in VATN mode and stably adjustthe average pretilt angle of the liquid crystal layer to 87° to 89.5°,which is a suitable angle in VATN mode, and further, more suppress theAC image sticking. It is preferable that the first constitutional unithas a side chain containing at least one photofunctional group selectedfrom the group consisting of a coumarin group, a cinnamate group, achalcone group, an azobenzene group, and a stilbene group. It ispreferable that the second constitutional unit has a side chaincontaining a steroid skeleton. The second constitutional unit may have aside chain having a structure in which three or four rings of1,4-cyclohexylene and/or 1,4-phenylene are linearly bonded to oneanother directly or with 1,2-ethylene there between. That is, the secondconstitutional unit may be the following unit. The unit has a side chainhaving a structure where three or four rings are linearly bonded to oneanother, and the three or four rings are each independently selectedfrom 1,4-cyclohexylene and 1,4-phenylene, and the three or four ringsare each independently bonded to one another through a single bond orwith 1,2-ethylene there between. In addition, it is preferable that thesecond constitutional unit has a side chain having a structure wherethree or four rings are linearly bonded to one another, two rings on theend of the three or four rings are 1,4-phenylene, and one or two ringson the main chain side of the three or four rings are each independentlyselected from 1,4-cyclohexylene and 1,4-phenylene, and the three or fourrings are bonded to one another through a single bond. It is preferablethat the polymer has at least one main chain structure selected from thegroup consisting of a polyamic acid, a polyimide, a polyamide, and apolysiloxane. It is preferable that each of the first and secondconstitutional units is derived from a diamine. It is preferable thatthe polymer is a copolymer obtain able by polymerizing a monomercomponent containing a diamine and at least one of an acid anhydride anda dicarboxylic acid.

The polymer may be a polyamide-imide polymer. In order to improve heatresistance and electrical characteristics of the alignment film, it ismore preferable that the polymer has a main chain structure of at leastone of a polyamic acid and a polyimide. That is, the polymer is morepreferably a copolymer obtain able by polymerizing a monomer componentcontaining a diamine and an acid anhydride.

In order to more effectively suppress the AC image sticking, it ispreferable that a ratio by weight (introduction ratio) of a monomercomponent of the second constitutional unit to a monomer component ofthe first constitutional unit is 4% to 40%, and more preferably 15% to40%. In order to more effectively suppress the AC image sticking andmore increase the average pretilt angle of the liquid crystal layer inVATN mode, a ratio by weight of a monomer component of the secondconstitutional unit to a monomer component of the first constitutionalunit is preferably 4% or more, and more preferably 15% or more. In orderto more effectively suppress the AC image sticking and to more decreasethe average pretilt angle of the liquid crystal layer in VATN mode, itis preferable that a ratio by weight of a monomer component of thesecond constitutional unit to a monomer component of the firstconstitutional unit is 40% or less.

It is preferable that the liquid crystal display device includes pixelsarranged in a matrix pattern, each of the pixels including a pixelelectrode and a common electrode, the pixel electrode being arranged ina matrix pattern on a liquid crystal layer side-surface of one of thepair of substrates, the common electrode being arranged on a liquidcrystal layer side-surface of the other substrate, wherein each of thepixel includes two or more domains adjacent to each other. According tosuch an embodiment, a boundary between adjacent two domains isredundantly exposed, and in such a part (doubly-exposed part), the ACimage sticking tends to be strongly generated. In the doubly-exposedpart, the pretilt angle of the liquid crystal molecules tends to vary.However, if the alignment film of the present invention is applied tothis embodiment, the AC image sticking and the variation in pretiltangle of the liquid crystal molecule in the doubly-exposed part can beeffectively suppressed, and the viewing angle can be increased. Inaddition, in order to increase the viewing angle in four directions, forexample, upper, lower, right, and left directions, it is preferable thatthe pixel has four domains.

Thus, it is preferable in the liquid crystal display device that eachpixel region is compartmentalized into some regions and the regions areseparately exposed (photoirradiated), and thereby alignment division isprovided for the device. VATN and ECB mode is preferable and VATN modeis particularly preferable as the multi-domain liquid crystal mode.

The above-mentioned various embodiments in the first liquid crystaldisplay device may be appropriately combined.

The present invention is also a liquid crystal display device including:a pair of substrates; a liquid crystal layer containing liquid crystalmolecules; and an alignment film, the liquid crystal layer beinginterposed between the pair of substrates, the alignment film beingarranged on a liquid crystal layer side-surface of at least one of thepair of substrates, wherein the alignment film includes a polymercontaining: a third constitutional unit having a structure derived froma photofunctional group; and a fourth constitutional unit having analignment functional group (herein after, also referred to as a “secondliquid crystal display device of the present invention”).

The second liquid crystal display device of the present invention isdescribed in more detail below.

According to the second liquid crystal display device, a liquid crystallayer containing liquid crystal molecules is interposed between a pairof substrates, and an alignment film is arranged on a liquid crystallayer-side surface of at least one of the pair of substrates.

The configuration of the second liquid crystal display device is notespecially limited as long as the second liquid crystal display deviceessentially includes such standard components a common liquid crystaldisplay device has.

The second liquid crystal display device may be a passive matrix liquidcrystal display device, but preferably an active matrix liquid crystaldisplay device. Thus, it is preferable that the second liquid crystaldisplay device includes pixels arranged in a matrix pattern,

the pixels including a pixel electrode and a common electrode,

the pixel electrode being arranged in a matrix pattern on a liquidcrystal layer side-surface of one of the pair of substrates, and

the common electrode being arranged on a liquid crystal layerside-surface of the other substrate.

From the same viewpoint as mentioned in the first liquid crystal displaydevice, according to the second liquid crystal display device, it ispreferable that the alignment film is arranged on liquid crystal layerside-surfaces of the both substrates, and further it is preferable thatthe second liquid crystal display device includes an alignment film onliquid crystal layer-side surfaces of the both substrates, and thealignment film includes a polymer containing: a third constitutionalunit having a structure derived from a photofunctional group; and afourth constitutional unit having an alignment functional group withouta photofunctional group and a structure derived from a photofunctionalgroup.

The alignment film includes a polymer containing: the thirdconstitutional unit having a structure derived from a photofunctionalgroup, and the fourth constitutional unit having an alignment functionalgroup without a structure derived from a photofunctional group. As aresult, the second liquid crystal display device can exhibit the sameeffects as in the first liquid crystal display device.

The structure derived from the photofunctional group is not especiallylimited, and preferably is at least one structure selected from thegroup consisting of a photofunctional group-bonding structure, adecomposition reaction, a photoisomerization structure, and aphotorealignment structure.

More specifically, the structure derived from the photofunctional groupis at least one structure selected from the group consisting of aphotofunctional group-bonding structure, a photofunctionalgroup-decomposition structure, a photofunctionalgroup-photoisomerization structure, and a photofunctionalgroup-photorealignment structure.

The photofunctional group-bonding structure is a structure resultingfrom bonding of photofunctional groups by photoirradiation. It ispreferable that this structure is formed through a crosslinking reaction(including a dimerization reaction).

The photofunctional group-decomposition structure is a structureresulting from decomposition of a photo-functional group byphotoirradiation.

The photofunctional group-photoisomerization structure is a structureresulting from isomerization of a photofunctional group byphotoirradiation. Accordingly, the third constitutional unit has, forexample, a structure obtained when a cis-(trans-)photofunctional groupis changed into its trans-(cis-)photofunctional group through anexcitation state by photoirradiation.

The photofunctional group-photorealignment structure is a structureresulting from photorealignment of a photofunctional group. Thephotorealignment means that a photofunctional group changes only itsdirection by photoirradiation without being isomerized. Accordingly, thethird constitutional unit has, for example, a structure obtained when acis-(trans-)photofunctional group changes its direction through anexcitation state without being isomerized by photoirradiation.

In the polymer, how the constitutional units are distributed is notespecially limited. The polymer may be any of an alternating copolymer,a block copolymer, a random copolymer, and a graft copolymer. Themolecular weight of the polymer is not especially limited, but it ispreferable that the polymer has a molecular weight suitable for use asthe alignment film, similarly to a polymer contained in a conventionalalignment film. The proportion of each constitutional unit in thepolymer is not especially limited, but a preferable ratio (% by weight)between the two constitutional units is mentioned below.

The alignment film is provided with an alignment treatment byphotoirradiation (preferably, UV light-irradiation). So it is preferablethat the alignment film is sensitive to light, particularly UV light,and more specifically, it is preferable that the alignment film reactsto light, particularly UV light with a smaller exposure energy and in ashort time. In order to shorten a tact time in the production process,the alignment film is preferably photoirradiated at an exposure energyof 100 mJ/cm² or less, and more preferably at an exposure energy of 50mJ/cm² or less. If the alignment film is provided with an alignmenttreatment by compartmentalizing each pixel region into some regions andseparately exposing the regions through a light-shielding mask(photomask) and the like, it is preferable that the alignment film isphotoirradiated at an exposure energy of 20 mJ/cm² or less.

Preferable embodiments of the second liquid crystal display device arementioned in more detail below.

In order to more uniformly align liquid crystal molecules, that is,suppress a variation in pretilt angle, it is preferable that the thirdconstitutional unit has at least one structure selected from the groupconsisting of a photofunctional group-bonding structure, aphotoisomerization structure, and a photorealignment structure.

It is preferable that the third constitutional unit has a side chainhaving a structure derived from a photofunctional group. It ispreferable that the fourth constitutional unit has a side chaincontaining an alignment functional group but not having a structurederived from a photofunctional group. As a result, the second liquidcrystal display device can more easily provided, and the AC imagesticking can be more effectively reduced.

The alignment functional group is not especially limited as long as itis a functional group capable of exhibiting a property of controllingalignment of liquid crystal molecules regardless of photoirradiation,and publicly known alignment functional groups such as a verticalalignment functional group and a horizontal alignment functional groupmay be used. Similarly to the first liquid crystal display device, thevertical alignment functional group is preferably a functional groupcapable of exhibiting a property of aligning liquid crystal moleculesvertically by rubbing or without any treatment, more preferably withoutany treatment, i.e., without the alignment treatment. The horizontalalignment functional group is preferably a functional group capable ofexhibiting a property of aligning liquid crystal molecules horizontallyby rubbing or without any treatment.

From the same viewpoint as in the first liquid crystal display device,according to the second liquid crystal display device, the followingembodiments are preferable. It is preferable that the third and fourthconstitutional units align the liquid crystal molecules in the samedirection. It is preferable that the alignment film uniformly controlsalignment of the liquid crystal molecules in a plane of the alignmentfilm. It is preferable that the alignment film is a vertical alignmentfilm that aligns liquid crystal molecules vertically. It is preferablethat the alignment film aligns the liquid crystal molecules in such away that an average pretilt angle of the liquid crystal layer is 87° ormore, and more preferably 87.5° or more. It is preferable that thealignment film aligns the liquid crystal molecules in such a way that anaverage pretilt angle of the liquid crystal layer is 89.5° or less, morepreferably 88.5° or less. It is preferable that the alignment filmaligns the liquid crystal molecules in such a way that an averagepretilt angle of the liquid crystal layer is 87° to 89.5°, morepreferably 87.5° to 88.5°. It is preferable that the fourthconstitutional unit has a side chain containing a vertical alignmentfunctional group. It is preferable that the third constitutional unithas a side chain having a structure derived from at least onephotofunctional group selected from the group consisting of a coumaringroup, a cinnamate group, a chalcone group, an azobenzene group, and astilbene group. It is preferable that the fourth constitutional unit hasa side chain containing a steroid skeleton. The fourth constitutionalunit may have a side chain having a structure where three or four ringsof 1,4-cyclohexylene and/or 1,4-phenylene are linearly bonded to oneanother directly or with 1,2-ethylene there between. That is, the fourthconstitutional unit may be the following unit. The unit has a side chainhaving a structure where three or four rings are linearly bonded to oneanother, and the three or four rings are each independently selectedfrom 1,4-cyclohexylene and 1,4-phenylene, and the three of four ringsare each independently bonded to one another through a single bond orwith 1,2-ethylene there between. In addition, it is more preferable thatthe second constitutional unit has a side having a structure where threeor four rings are linearly bonded to one another, and two rings on theend of the three of four rings are 1,4-phenylene, and one or two ringson the main chain side of the three or four rings are each independentlyselected from 1,4-cyclohexylene and 1,4-phenylene, and the three or fourrings are bonded to one another through a single bond. It is preferablethat the polymer has at least one main chain structure selected from thegroup consisting of a polyamic acid, a polyimide, a polyamide, and apolysiloxane. It is preferable that each of the third and fourthconstitutional units is derived from a diamine. It is preferable thatthe polymer is a copolymer obtain able by polymerizing a monomercomponent containing a diamine and at least one of an acid anhydride anda dicarboxylic acid. The polymer may be a polyimide-amide polymer. It ismore preferable that the polymer has a main chain structure of at leastone of a polyamic acid and a polyimide. The polymer is more preferably acopolymer obtain able by polymerizing a monomer component containing adiamine and an acid anhydride. It is preferable that a ratio by weightof a monomer component of the fourth constitutional unit to a monomercomponent of the third constitutional unit is preferably 4% or more, andmore preferably 15% or more. It is preferable that a ratio by weight ofa monomer component of the fourth constitutional unit to a monomercomponent of the third constitutional unit is 40% or less. It ispreferable that a ratio by weight (introduction ratio) of a monomercomponent of the fourth constitutional unit to a monomer component ofthe third constitutional unit is 4% to 40%, and more preferably 15% to40%. It is preferable that the liquid crystal display device includespixels arranged in a matrix pattern, the pixels including a pixelelectrode and a common electrode, the pixel electrode being arranged ina matrix pattern on a liquid crystal layer side-surface of one of thepair of substrates, the common electrode being arranged on a liquidcrystal layer side-surface of the other substrate, wherein the pixelsinclude two or more domains adjacent to each other. It is preferablethat the pixel has four domains. It is preferable in the liquid crystaldisplay device that each pixel region is divided into some regions andthe regions are separately exposed (photoirradiated), and therebyalignment division is provided for the device. VATN and ECB mode ispreferable and VATN mode is particularly preferable as the multi-domainliquid crystal mode. Thus, the various embodiments in the first liquidcrystal display device can be appropriately applied to the second liquidcrystal display device.

The above-mentioned various embodiments in the second liquid crystaldisplay device are appropriately combined.

The present invention is also a polymer for alignment film materials,included in an alignment film material for the alignment film includedin the above-mentioned liquid crystal display devices. According tothis, the AC image sticking in a liquid crystal display device can besuppressed, and a liquid crystal display device with excellent displayqualities can be provided. In addition, the alignment film can beprovided with an alignment treatment by a photo-alignment method, and aliquid crystal display device can be easily provided. In addition, acoating property of an alignment film material can be improved.

The various embodiments of the polymers in the first and second liquidcrystal display devices of the present invention can be appropriatelyapplied to the polymer for alignment film materials of the presentinvention.

EFFECT OF THE INVENTION

According to the liquid crystal display device and the polymer foralignment film materials of the present invention, generation of the ACimage sticking can be suppressed.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is mentioned in more detail below with referenceto Embodiments using drawings, but not limited to only theseEmbodiments. In the present Embodiment, a VATN liquid crystal displaydevice is mentioned in detail, but the present invention can be appliedto horizontal alignment TN, IPS, and ECB devices, and suppression of theAC image sticking is expected. That is, if the present invention isapplied to a horizontal alignment device, the following copolymer may beused as a polymer included in an alignment film material. The copolymeris obtain able by polymerizing a constitutional unit having a side chainnot containing a vertical alignment functional group (for example, adiamine) or a constitutional unit having a side chain containing ahydrophilic functional group or a horizontal alignment functional group(for example, a diamine) with a constitutional unit containing ahorizontal alignment photofunctional group (for example, a diamine).

Embodiment 1

The present Embodiment is mentioned in the following order: 1. alignmentfilm material; 2. preparation method of alignment film; 3. basicoperations of liquid crystal display device; 4. production method ofliquid crystal display device; and 5. evaluation test of AC imagesticking

1. Alignment Film Material

The alignment film material of the present Embodiment includes a polymeressentially containing a first constitutional unit and a secondconstitutional unit. The first constitutional unit exhibits a propertyof controlling alignment of liquid crystal molecules byphotoirradiation. The second constitutional unit exhibits a property ofcontrolling alignment of liquid crystal molecules regardless ofphotoirradiation. More particularly the first constitutional unit has aside chain containing a photofunctional group, and the secondconstitutional unit has a side chain containing a vertical alignmentfunctional group. Thus, the side chain of the second constitutional unitcontains a functional group that aligns liquid crystal moleculesvertically, that is, a functional group that aligns the liquid crystalmolecules substantially vertically to the alignment film surface. Theessential constitutional units (the first constitutional unit and thesecond constitutional unit) of the polymer align liquid crystalmolecules in the same direction (the same to such an extent that thedevice can be driven in VATN mode). The alignment film of the presentEmbodiment, which is obtain able by providing a film with an alignmenttreatment by photoirradiation, the film being formed from the alignmentfilm material of the present Embodiment, can align liquid crystalmolecules uniformly (to such an extent that the device can be driven inVATN mode) in the alignment film plane. Thus, the alignment film of thepresent Embodiment is a vertical alignment film that controls alignmentof liquid crystal molecules substantially vertically to the alignmentfilm surface. It is preferable that the alignment film controlsalignment of the liquid crystal molecules in such a way that the averagepretilt angle of the liquid crystal layer is 87° to 89.5°, morepreferably 87.5° to 88.5°.

Each of the essential constitutional units is derived from a diamine.That is, the diamine is a monomer component of the essentialconstitutional units. The polymer of the present Embodiment is acopolymer obtain able by polymerizing a monomer component containing adiamine and an acid anhydride. The polymer of the present Embodiment hasa main chain structure of at least one of a polyamic acid and apolyimide. Thus, the liquid crystal display device including thealignment film formed from the alignment film material of the presentEmbodiment can be effectively driven in VATN mode, and the averagepretilt angle of the liquid crystal layer can be stably controlled to87° to 89.5° (more preferably 87.5° to 88.5°), which is preferable inVATN mode. In addition, the AC image sticking is effectively suppressed.

The polymer of the present Embodiment is mentioned with reference toFIG. 1. FIG. 1 shows a basic structure of the polymer included in thealignment film material in accordance with the present Embodiment. InFIG. 1, the part encircled by the solid line is a unit derived from anacid anhydride (acid anhydride unit); the part encircled by the dashedline is a unit derived from a diamine for a photo-alignment film, i.e.,a diamine having a side chain 21 containing a photofunctional group(photo-alignment diamine unit); and the part encircled by thedashed-dotted line is a unit derived from a diamine for a verticalalignment film, i.e., a diamine having a side chain 22 containing avertical alignment functional group (vertical alignment diamine unit).As shown in FIG. 1, the polymer of the present Embodiment is a copolymerobtain able by polymerizing two diamines that are monomer components ofthe first and second constitutional units with an acid anhydride. One ofthe two diamines is a diamine having a side chain containing aphotofunctional group and the other is a diamine having a side chaincontaining a vertical alignment functional group. The polymer of thepresent Embodiment is a polyamic acid or a polyimide constituted by acidanhydride unit and the photo-alignment diamine unit or the verticalalignment diamine unit, alternately arranged. According to a polymer fora conventional alignment film, if the alignment film is aphoto-alignment film, the polymer is a polyamic acid or a polyimideconstituted by an acid anhydride unit and a photo-alignment diamineunit, alternately arranged, and if the alignment film is a verticalalignment film, the polymer is a polyamic acid or a polyimideconstituted by an acid anhydride unit and a photo-alignment diamineunit, alternately arranged.

The first constitutional unit contains at least one photofunctionalgroup selected from the group consisting of a cinnamate group (thefollowing formula (1)), a culcon group (the following formula (2)), anazobenzene group (the following formula (3)), a stilbene group (thefollowing formula (4)), a cinnamoyl group, and a coumarin group. Thesephotofunctional groups undergo any of a crosslinking reaction (includinga dimerization reaction), isomerization, photorealignment, or a complexreaction thereof by photoirradiation, thereby exhibiting a function ofaligning liquid crystal molecules that are positioned on the alignmentfilm surface in a desired direction depending on photoirradiationconditions such as an irradiation angle. A coumarin derivative includesa compound represented by the following formula (5), for example.Particularly, it is preferable that the first constitutional unit has aside chain containing at least one photofunctional group selected fromthe group consisting of a cinnamate group (absorption wavelength (λmax)of 270 nm), a culcon group (absorption wavelength (λmax) of 300 nm), anazobenzene group (absorption wavelength (λmax) of 350 nm), and astilbene group (absorption wavelength (λmax) of 295 nm). According tothis, the liquid crystal display device of the present invention can beeffectively driven in VATN mode, and the average pretilt angle of theliquid crystal layer can be stably controlled within 87° to 89.5° (morepreferably, 87.5° to 88.5°), which is a preferable range for VATN mode.In addition, the AC image sticking is effectively suppressed. Thesephotofunctional groups may be used singly or in combination of two ormore species of them.

The second constitutional unit may contain a vertical functional groupincluded in a conventional vertical alignment film. In particular, thesecond constitutional unit is preferably derived from a diaminerepresented by the following formula (7), (8), or (9). These diaminesmay be used singly or in combination of two or more species of them.

in the formula (7),

X representing a single bond, —O—, —CO—, —COO—, —OCO—, —NHCO—, —CONH—,—S—, or an arylene group; and

R⁴ representing an alkyl group with 10 to 20 carbon atoms, a monovalentorganic group having an alicyclic skeleton with 4 to 40 carbon atoms,and a fluorine atom-containing monovalent organic group with 6 to 20carbon atoms.

in the formula (8),

X representing a single bond, —O—, —CO—, —COO—, —OCO—, —NHCO—, —CONH—,—S—, or an arylene group; and

R⁵ representing an alicyclic skeleton-containing divalent organic groupwith 4 to 40 carbon atoms.

in the formula (9),

A¹, A², and A³ being each independently 1,4-cyclohexylene or1,4-phenylene;

A⁴ representing 1,4-cyclohexylene, 1,4-phenylene or a single bond;

B¹, B², and B³ being each independently a single bond or 1,2-ethylene;

R⁶ representing an alkyl with 1 to 20 carbon atoms and one —CH₂— in thealkyl may be substituted with —O—.

In the formula (7), examples of the alkyl group with 10 to 20 carbonatoms, represented by R⁴ include: an n-decyl group, an n-dodecyl group,an n-pentadecyl group, an n-hexadecyl group, an n-octadecyl group, andan n-eicosyl group.

Examples of the organic group having an alicyclic skeleton with 4 to 40carbon atoms, represented by R⁴ in the formula (7) and R⁵ in the formula(8), include: a group containing an alicyclic skeleton derived fromcycloalkanes such as cyclobutane, cyclopentane, cyclohexane, andcyclodecane; steroid skeleton-containing groups such as cholesterol andcholestanol; and bridged alicyclic skeleton-containing groups such asnorbornene and adamantane. Among them, the steroid skeleton-containinggroups are particularly preferable. The organic group having analicyclic skeleton may be substituted with a halogen atom, preferably afluorine atom, or a fluoroalkyl group, preferably a trifluoromethylgroup.

Examples of the fluorine atom-containing group with 6 to 20 carbonatoms, represented by R⁴ in the formula (7), include groups obtained bysubstituting some or all of hydrogen atoms in the following organicgroups with a fluorine atom or a fluoroalkyl group such as atrifluoromethyl group. The organic groups are: straight-chain alkylgroups with 6 or more carbon atoms, such as an n-hexyl group, an n-octylgroup, and an n-decyl group; alicyclic hydrocarbon groups with 6 or moreof carbon atoms, such as a cyclohexyl group and a cyclooctyl group; andaromatic hydrocarbon groups with 6 or more of carbon atoms, such as aphenyl group and a biphenyl group.

Examples of X in the formulae (7) and (8) include: a single bond, —O—,—CO—, —COO—, —OCO—, —NHCO—, —CONH—, —S—, or an arylene group. Examplesof the arylene group include a phenylene group, a tolylene group, abiphenylene group, a naphthylene group. Among them, —O—, —COO—, and—OCO— are still more preferable.

Specific examples of the diamine containing the group represented by theformula (7) preferably include:

dodecanoxy-2,4-diaminobenzene,

pentadecanoxy-2,4-diaminobenzene,

hexadecanoxy-2,4-diaminobenzene,

octadecanoxy-2,4-diaminobenzene, and compounds represented by thefollowing formulae (10) to (15).

Specific examples of the diamine containing the group represented by theformula (8) preferably include: diamines represented by the followingformulae (16) to (18).

In the formula (9), R⁶ is any straight or branched alkyl selected fromalkyls with 1 to 20 carbon atoms. One —CH₂— in the alkyl may besubstituted with —O—. Specific examples of the alkyls include: methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl,undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl, octadecyl, nonadecyl, eicosyl, isopropyl, isobutyl,sec-butyl, t-butyl, isopentyl, neopentyl, t-pentyl, 1-methyl pentyl,2-methyl pentyl, 3-methyl pentyl, 4-methyl pentyl, isohexyl, 1-ethylpentyl, 2-ethyl pentyl, 3-ethyl pentyl, 4-ethyl pentyl, 2,4-dimethylhexyl, 2,3,5-triethyl heptyl methoxy, ethoxy, propyl oxy, butyloxy,pentyl oxy, hexyl oxy, methoxy methyl, methoxy ethyl, methoxy propyl,methoxy butyl, methoxy pentyl, methoxyhexyl, ethoxymethyl, ethoxyethyl,ethoxypropyl, ethoxy butyl, ethoxy pentyl, ethoxy hexyl, hexyloxymethyl, hexyl oxyethyl, hexyl oxypropyl, hexyl oxybutyl, hexyloxypentyl, hexyl oxyhexyl. Among them, propyl, butyl, pentyl, hexyl,heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl,pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, andthe like are preferably mentioned.

In the formula (9), B¹, B², and B³ each independently represents asingle bond or 1,2-ethylene. The number of 1,2-ethylene in the formula(9) is preferably 0 or 1.

In the formula (9), compounds containing some of R⁶, A¹, A², A³, A⁴, B¹,B², and B³ in a combination shown in the following Tables 1 to 3 areparticularly preferable. In Tables, B represents 1,4-phenylene; Chrepresents 1,4-cyclohexylene; — represents a single bond; and Erepresents 1,2-ethylene. Cis-1,4-cyclohexylene, trans-1,4-cyclohexylenemay be mixed, and trans-1,4-cyclohexylene is preferred.

TABLE 1 No. R⁶ A¹ A² A³ A⁴ B¹ B² B³ 1 Me Ch Ch B — — — — 2 n-C₃H₇ Ch ChB — — — — 3 n-C₅H₁₁ Ch Ch B — — — — 4 n-C₇H₁₅ Ch Ch B — — — — 5 n-C₁₂H₂₅Ch Ch B — — — — 6 n-C₁₆H₃₂ Ch Ch B — — — — 7 n-C₂₀H₄₁ Ch Ch B — — — — 8n-C₃H₇ Ch Ch B — E — — 9 n-C₅H₁₁ Ch Ch B — E — — 10 n-C₇H₁₅ Ch Ch B — E— — 11 n-C₁₂H₂₅ Ch Ch B — E — — 12 n-C₁₅H₃₁ Ch Ch B — E — — 13 n-C₁₉H₃₉Ch Ch B — E — — 14 n-C₃H₇ Ch Ch B — — E — 15 n-C₅H₁₁ Ch Ch B — — E — 16n-C₇H₁₅ Ch Ch B — — E — 17 n-C₁₂H₂₅ Ch Ch B — — E — 18 n-C₁₄H₂₉ Ch Ch B— — E — 19 n-C₈H₁₈O Ch Ch B — — — — 20 n-C₁₆H₃₂O Ch Ch B — — — — 21n-C₁₂H₂₅O Ch Ch B — E — — 22 n-C₅H₁₁ Ch B Ch — — — — 23 n-C₇H₁₅ Ch B Ch— — — — 24 n-C₁₂H₂₅ Ch B Ch — — — —

TABLE 2 No. R⁶ A¹ A² A³ A⁴ B¹ B² B³ 25 n-C₅H₁₁ B Ch Ch — — — — 26n-C₇H₁₅ B Ch Ch — — — — 27 n-C₁₂H₂₅ B Ch Ch — — — — 28 n-C₂₀H₄₁ B Ch Ch— — — — 29 n-C₃H₇ B Ch Ch — E — — 30 n-C₇H₁₅ B Ch Ch — E — — 31 n-C₅H₁₁B Ch Ch — — E — 32 n-C₁₈H₃₇ B Ch Ch — — E — 33 n-C₅H₁₁ Ch B B — — — — 34n-C₇H₁₅ Ch B B — — — — 35 n-C₁₂H₂₅ Ch B B — — — — 36 n-C₁₆H₃₂ Ch B B — —— — 37 n-C₂₀H₄₁ Ch B B — — — — 38 n-C₅H₁₁ Ch B B — E — — 39 n-C₇H₁₅ Ch BB — E — — 40 n-C₃H₇ B B Ch — — — — 41 n-C₇H₁₅ B B Ch — — — — 42 n-C₁₂H₂₅B B Ch — — — — 43 n-C₅H₁₁ B B B — — — — 44 n-C₇H₁₅ B B B — — — — 45n-C₅H₁₁ Ch Ch Ch B — — — 46 n-C₇H₁₅ Ch Ch Ch B — — — 47 n-C₁₂H₂₅ Ch ChCh B — — — 48 n-C₃H₇ Ch Ch B B — — —

TABLE 3 No. R⁶ A¹ A² A³ A⁴ B¹ B² B³ 49 n-C₅H₁₁ Ch Ch B B — — — 50n-C₇H₁₅ Ch Ch B B — — — 51 n-C₁₄H₂₉ Ch Ch B B — — — 52 n-C₂₀H₄₁ Ch Ch BB — — — 53 n-C₃H₇ Ch Ch B B E — — 54 n-C₇H₁₅ Ch Ch B B E — — 55 n-C₁₂H₂₅Ch Ch B B E — — 56 n-C₃H₇ Ch Ch B B — E — 57 n-C₅H₁₁ Ch Ch B B — E — 58n-C₇H₁₅ Ch Ch B B — E — 59 n-C₇H₁₅ B B Ch Ch — — — 60 n-C₁₄H₂₉ B B Ch Ch— — — 61 n-C₂₀H₄₁ B B Ch Ch — — — 62 n-C₅H₁₁ B B Ch Ch — E — 63 n-C₇H₁₅B B Ch Ch — E — 64 n-C₇H₁₅ B B Ch Ch — — E 65 n-C₁₄H₂₉ B B Ch Ch — — E66 n-C₅H₁₁ B Ch Ch Ch — — — 67 n-C₇H₁₅ B Ch Ch Ch — — — 68 n-C₅H₁₁ Ch BB B — — — 69 n-C₇H₁₅ Ch B B B — — —

Specific examples of the diamine containing the group represented by theformula (9) preferably include a diamine represented by the formula(19).

Thus, it is preferable that the second constitutional unit has a sidechain having a steroid skeleton or a side chain having a structure inwhich three or four rings selected from 1,4-cyclohexylene and1,4-phenylene are linearly bonded to one another directly or with1,2-ethylene there between. Thus, the liquid crystal display device ofthe present invention can be effectively driven in VATN mode, and theaverage pretilt angle of the liquid crystal layer can be stablycontrolled within the range of 87° to 89.5° (preferably 87.5° to 88.5°),which is a preferable range for VATN mode. In addition, the AC imagesticking is effectively suppressed.

The following acid anhydrides are preferable as the acid anhydride usedfor the copolymer of the present Embodiment. An acid anhydride (PMDA)represented by the formula (20), an acid anhydride (CBDA) represented bythe formula (21), an acid anhydride (BPDA) represented by the formula(22), an acid anhydride (exoHDA) represented by the formula (23), anacid anhydride (BTDA) represented by the formula (24), an acid anhydride(TCA) represented by the formula (25), an acid anhydride (NDA)represented by the formula (26). These acid anhydrides may be usedsingle or in combination of two or more species of them.

The copolymer of the present Embodiment may be a polyamide, apolyamide-imide, or a polysiloxane. That is, the copolymer of thepresent Embodiment may have a main chain structure of a polyamide. Inthis case, the copolymer of the present Embodiment can be formed bypolymerizing the first and second constitutional units, with adicarboxylic acid. The copolymer of the present Embodiment may have amain chain structure of a polysiloxane, i.e., a main chain structurecontaining a siloxane bond (≡Si—O—Si≡).

The copolymer of the present Embodiment may be constituted by the firstconstitutional unit containing a photofunctional group that undergoes adecomposition reaction by photoirradiation. In order to suppress avariation in pretilt angle, it is preferable that the firstconstitutional unit includes a photofunctional group that undergoes anyone of a crosslinking reaction (including a dimerization reaction),isomerization, and photorealignment, or a complex reaction thereof byphotoirradiation, as mentioned above. Polyvinyl alcohols, polyamides,and polyimides, and the like, are mentioned as an alignment filmmaterial that undergoes a photodecomposition reaction (decompositionreaction generated by light), thereby providing liquid crystals with apretilt angle.

The reason why the AC image sticking is suppressed in a liquid crystaldisplay device including an alignment film formed from the alignmentfilm material of the present Embodiment is further mentioned withreference to FIGS. 2 and 3. FIG. 2 is a cross-sectional viewschematically showing a vicinity of the alignment film surface in theliquid crystal display device in accordance with Embodiment 1 andexplaining a mechanism for suppressing AC image sticking due toside-chain deformation. FIG. 2( a) shows an initial state. FIG. 2( b)shows a state where an electrical field is applied to the liquid crystallayer. FIG. 2( c) shows a state where the application of the electricalfield to the liquid crystal layer is stopped. FIG. 3 is across-sectional view schematically showing a vicinity of the alignmentfilm surface in the liquid crystal display device in accordance withEmbodiment 1 and explaining a mechanism for suppressing AC imagesticking cause by the liquid crystal adsorption. FIG. 3( a) shows aninitial state. FIG. 3( b) shows a state where an electrical field isapplied to the liquid crystal layer. FIG. 3( c) shows a state where theapplication of the electrical field to the liquid crystal layer isstopped.

The mechanism for suppressing AC image sticking due to side-chaindeformation is mentioned below. As shown in FIG. 2( a), in the initialstate, similarly to a conventional case, liquid crystal molecules 11contained in a liquid crystal layer 20 and side chains of an alignmentfilm 10 (a side chain 21 containing a photofunctional group and a sidechain 22 containing a vertical alignment functional group) interact witheach other, and the molecules 11 are pre-tilted. As shown in FIG. 2( b),when an electrical field is applied to the liquid crystal layer 20, theside chain 22 containing a vertical alignment functional groupsuppresses the side chain 21 containing a photofunctional group frombeing inclined by bending deformation of the molecules 11 (suppressionof elastic deformation, steric hindrance). As shown in FIG. 2( c), whenthe application of the electrical field to the liquid crystal layer 20is stopped, the side chains of the alignment film 10 are hardlyinclined. As a result, it is considered that a change in the tilt angleof the liquid crystal layer 20 could be suppressed, which results insuppression of the AC image sticking.

The mechanism for suppressing AC image sticking due to liquid crystaladsorption is mentioned below. Even if light such as UV light isradiated to the alignment film 10, a portion where the liquid crystalmolecules 11 adsorb to a main chain 25 is decreased as shown in FIG. 3(a) in the initial state, due to steric hindrance of the side chain 22containing a vertical alignment functional group. As shown in FIG. 3(b), also when an electrical field is applied to the liquid crystal layer20, steric hindrance of the side chain 22 containing a verticalalignment functional group suppresses the liquid crystal molecules 11from passing through the side chains of the alignment film 10 (the sidechain 21 containing a photofunctional group and the side chain 22containing a vertical alignment functional group) and then aligning withthe adsorbing liquid crystal molecules (the liquid crystal moleculesthat have adsorbed to the main chain since the initial state). Thus,generation of the liquid crystal molecules that adsorb between the mainchains and of the liquid crystal molecules that align with the absorbingliquid crystal molecules can be suppressed. As a result, it isconsidered that when the application of the electric field to the liquidcrystal layer 20 is stopped, a change in the tilt angle of the liquidcrystal layer 20 could be suppressed as shown in FIG. 3( c), whichresults in suppression of the AC image sticking.

Compared with conventional photo-alignment films, an improvement incoating properties of the alignment film material of the presentEmbodiment when the material is printed by spin coating, flexography,ink-jet printing, and the like can be expected. If the above-mentionedphoto-alignment diamine unit contains a fluorine atom at an end of itsside chain with the aim of improving a VHR, the unit shows highhydrophobicity. That is, a homopolymer of a conventional photo-alignmentdiamine unit commonly exhibits insufficient coating properties for asubstrate. In contrast, the copolymer of the present Embodiment,obtained by copolymerizing the photo-alignment diamine unit and thevertical alignment diamine unit, contains the photo-alignment diamineunit in a smaller amount, and so a proportion of the fluorine in thepolymer can be decreased. In addition, the vertical alignment diamineunit generally has lower hydrophobicity than that of fluorine.Accordingly, the coating properties for a substrate can be more improvedas the introduction ratio of the vertical alignment diamine unit isincreased.

The present invention can be applied to horizontal alignment mode suchas TN, ECB, and IPS mode. In this case, the AC image sticking can besuppressed using a horizontal alignment film that includes a copolymerof an imide derivative, an amide derivative, and the like, containing aphotofunctional group with an imide derivative, an amide derivative, andthe like, not containing a photofunctional group.

2. Preparation Method of Alignment Film

A preparation method of the alignment film of the present Embodiment ismentioned below.

First, the monomer components of the first and second constitutionalunits are copolymerized with an acid anhydride by a publicly knownmethod.

A varnish for applying (printing) the polymer to a substrate isprepared. The varnish preferably includes a mixed solvent containingsolvents such as γ-butyl lactone (BL), N-methylpyrrolidone (NMP), butylcellosolve (BC), diethyl ether dibutyl glycol (DEDG), diisobutyl ketone(DIBK), and dipentyl ether (DPE).

The varnish is applied to a substrate. Spin coating, flexography,ink-jet printing, and the like, are preferable for the application.

After being printed on the substrate, the varnish is pre-baked with ahot plate for pre-baking and then post-baked with a hot plate forpost-baking. In the pre-baking and post-baking, the temperature andheating time may be appropriately determined. The thickness of thealignment film of the present Embodiment may be appropriatelydetermined.

The alignment film of the present Embodiment may be formed by aso-called two-layered treatment or hybrid treatment. A residual DC hasbeen thought to be a main cause of the image sticking in liquid crystaldevices. An increase in thickness (volume) of an alignment filmcontributes to an increase in the residual DC. So a decrease inthickness (volume) of an alignment film leads to a decrease in theresidual DC. In order to prevent coating defects in a step of printingan alignment film in a panel production, the alignment film needs tokeep a certain thickness, e.g., 60 nm or more. In view of thesecircumstances, a so-called two-layered treatment or hybrid treatment canbe employed. If a varnish containing a polymer for a horizontalalignment film and a polymer for a vertical alignment film in a specificratio (for example, 50:50 to 10:90) is applied to a substrate, phaseseparation occurs between the two polymers immediate after the varnishcoating or in the process of baking the applied alignment film. Byutilizing this action, the horizontal alignment film is formed on thesubstrate side and the vertical alignment film is formed on the liquidcrystal layer side. As a result, the volume of the alignment filmexposing on the liquid crystal layer side is decreased, and the residualDC and the image sticking due to the residual DC can be reduced. Also inthe present Embodiment, the above-mentioned treatment may be employed,if necessarily. Thus, a liquid crystal display device in which both ofthe image sticking due to the residual DC and the image sticking in ACmode less occur can be provided.

Next, the alignment film formed on the substrate is provided with analignment treatment by photoirradiation. Conditions of the irradiationto the alignment film may be appropriately determined. It is preferablethat the alignment film is irradiated with light including UV light, andit is more preferable that the alignment film is irradiated with UVlight. In order to shorten a tact time in the production process, thealignment film is irradiated with light at an exposure energy of 100mJ/cm² or less, and more preferably 50 mJ/cm² or less. If the alignmentfilm is provided with an alignment treatment by compartmentalizing eachpixel region into some regions and separately exposing the regionsthrough a light-shielding mask (photomask) and the like, it ispreferable that the alignment film is irradiated with light at anexposure energy of 20 mJ/cm² or less. Other irradiation conditions (forexample, existence of polarized light, irradiation angle) may beappropriately determined.

Thus, the alignment film of the present Embodiment is formed andprovided with the alignment treatment. As a result, the alignment filmof the present Embodiment has a structure derived from a photofunctionalgroup, preferably at least one structure selected from the groupconsisting of a photofunctional group-bonding structure, aphotoisomerization structure, and a photo-alignment structure. Further,the alignment film provides liquid crystal molecules with asubstantially uniform pretilt angle in the alignment film plane.

3. Basic Operation of Liquid Crystal Display Device

The basic operation of the liquid crystal display device of the presentEmbodiment is mentioned below.

FIG. 4 is a perspective view schematically showing a relationshipbetween a photo-alignment treatment direction and a pretilt direction ofa liquid crystal molecule in accordance with Embodiment 1. FIG. 5( a) isa plan view schematically showing a director alignment of liquid crystalin one pixel (one sub-pixel); and directions of photo-alignmenttreatment for a pair of substrates (upper and lower substrates) in thecase that the liquid crystal display device in Embodiment 1 is amono-domain device. FIG. 5( b) is a schematic view showing directions ofabsorption axes of polarization plates arranged in the liquid crystaldisplay device shown in FIG. 5( a). FIG. 5( a) shows a state where thephoto-alignment treatment directions are perpendicular to each otherbetween a pair of substrates and an AC voltage not lower than athreshold voltage is applied between the pair of substrates. In FIG. 5(a), the solid arrow shows a direction of photo-irradiation (a directionof photo-alignment treatment) for a lower substrate; and the dottedarrow shows a direction of photo-irradiation (a direction ofphoto-alignment treatment) for an upper substrate. FIG. 6 is a plan viewschematically showing a director alignment of liquid crystal in onepixel (one sub-pixel); and directions of photo-alignment treatment for apair of substrates (upper and lower substrates) in the case that theliquid crystal display device in Embodiment 1 is a mono-domain device.FIG. 6( a) is a plan view schematically showing a director alignment ofliquid crystal in one pixel (one sub-pixel); and directions ofphoto-alignment treatment for a pair of substrates in the case that theliquid crystal display device in Embodiment 1 is a mono-domain device.FIG. 6( b) is a schematic view showing directions of absorption axes ofpolarization plates arranged in the liquid crystal display device shownin FIG. 6( a). FIG. 6( a) shows a state where photo-alignment treatmentdirections are parallel and opposite to each other between the pair ofsubstrates and an AC voltage not lower than a threshold voltage isapplied between the pair of substrates. In FIG. 6( a), the solid arrowshows a direction of photoirradiation (a direction of photo-alignmenttreatment) for a lower substrate; and the dotted arrow shows a directionof photoirradiation (a direction of photo-alignment treatment) for anupper substrate. FIG. 7 is a cross-sectional view schematically showinga first arrangement relationship between substrates and a photomask in aphoto-alignment treatment process in accordance with Embodiment 1 wherealignment division is performed by proximity exposure using an alignmentmask. FIG. 8 is a cross-sectional view schematically showing a secondarrangement relationship between substrates and a photomask in aphoto-alignment treatment process in accordance with Embodiment 1 wherealignment division is performed by proximity exposure using an alignmentmask. FIG. 9( a) is a plan view schematically showing: the averagedirector alignment of liquid crystal in one pixel (one sub-pixel);directions of photo-alignment treatment for a pair of substrates (upperand lower substrates); and a arrangement pattern of a multi-domain, inthe case that the liquid crystal display device in Embodiment 1 is afour-domain device. FIG. 9( b) is a schematic view showing directions ofabsorption axes of polarization plates in the liquid crystal displaydevice shown in FIG. 9( a). FIG. 9( a) shows a state where an AC voltagenot lower than a threshold voltage is applied between the pair ofsubstrates. In FIG. 9( a), the solid arrow shows a direction ofphotoirradiation (a direction of photo-alignment treatment) to a lowersubstrate (driving element substrate); and the dotted arrow shows adirection of photoirradiation (a direction of photo-alignment treatment)to an upper substrate (color filter substrate).

The operation principle of the liquid crystal display device of thepresent Embodiment is mentioned with reference to FIGS. 4 to 9.

According to the liquid crystal display device of the presentEmbodiment, a liquid crystal layer containing liquid crystal moleculeswith negative dielectric anisotropy is interposed between a pair ofsubstrates (upper and lower substrates). Each of the pair of substratesincludes an insulating transparent substrate such as a glass substrate.On a liquid crystal layer side-surface of each substrate, a transparentelectrode is formed. On the transparent electrode, the above-mentionedvertical alignment film is formed. One of the pair of substratesfunctions as a driving element substrate (for example, TFT substrate)having a driving element (a switching element) formed in every pixel(every sub-pixel). The other functions as a color filter substratehaving a color filter formed to face each pixel (each sub-pixel) of thedriving element substrate. That is, in the liquid crystal display deviceof the present Embodiment, one of the pair of substrates (upper andlower substrates) is a color filter substrate and the other is a drivingelement substrate. In the driving element substrate, the transparentelectrode that is connected to the driving element and arranged in amatrix pattern functions as a pixel electrode. In the color filtersubstrate, the transparent electrode that is uniformly formed over theentire display region functions as a counter electrode (commonelectrode). Polarization plates are each arranged on a surface on theside opposite to the liquid crystal layer side of each substrate in aCross-Nicol state for example. Between the pair of substrates, a cellgap controlling member (spacer) for controlling a constant cell gap isarranged at a specific position (in non-display region). The materialfor the substrates, the transparent electrodes, and the liquid crystalmolecules, and the like, are not especially limited.

As shown in FIG. 4, the alignment film 10 provides liquid crystalmolecules 11 with a pretilt angle in an UV-irradiation direction ifbeing irradiated with UV light polarized parallel to an incident face(shown by the outline arrow in FIG. 4), for example, from a directionmaking an angle of 40° with the normal direction of the substrate face.The alignment film 10 may be exposed by shot exposure or scanningexposure. That is, the alignment film 10 may be irradiated with UV lightwith the substrate and a light source being fixed. As shown in thedotted arrow in FIG. 4, the alignment film 10 may be irradiated with UVlight by being scanning with UV light in the UV scanning direction.

In the liquid crystal display device of the present Embodiment, exposurefor the alignment films and attachment of the pair of substrates (upperand lower substrates 12) may be performed so that a direction ofphotoirradiation to one of the pair of substrates is substantiallyperpendicular to a direction of photoirradiation to the other substratewhen the pair of substrates are viewed in plane as shown in FIG. 5( a).Liquid crystal molecules near the alignment films arranged on the upperand lower substrates 12 may have substantially the same pretilt angle.Liquid crystal materials free from a chiral material may be injectedbetween the substrates as a liquid crystal layer. In this case, byapplying an AC voltage not less than a threshold voltage value betweenthe upper and lower substrates 12, liquid crystal molecules twist 90° inthe normal direction of the substrate plane between the upper and lowersubstrates 12, and as shown in FIG. 5, the average liquid crystaldirector alignment 17 under AC voltage application is in a directionbisecting an angle made by the directions of photoirradiation to theupper and lower substrates 12 when the substrates are viewed in plane.As shown in FIG. 5( b), a direction of an absorption axis of apolarization plate (upper polarization plate) arranged on the uppersubstrate side is the same as a direction of photoirradiation to theupper substrate, and a direction of an absorption axis of the otherpolarization plate (lower polarization plate) arranged on the lowersubstrate side is the same as a direction of photoirradiation to thelower substrate. The liquid crystal display device of the presentinvention, produced through the above-mentioned alignment treatment forthe alignment films and arrangement of the polarization plates, is aso-called VATN device.

In the liquid crystal display device of the present Embodiment, exposurefor the alignment films and attachment of the substrates may beperformed so that directions of photoirradiation to the upper and lowersubstrates 12 are substantially parallel and opposite to each other whenthe substrates are viewed in plane, as shown in FIG. 6( a). Liquidcrystal molecules near the alignment films arranged on the upper andlower substrates 12 may have substantially the same pretilt angle.Liquid crystal materials free from a chiral material may be injectedbetween the substrates as a liquid crystal layer. In this case, when novoltage is applied between the upper and lower substrates 12, liquidcrystal molecules near the interface between the liquid crystal layerand the upper and lower substrates 12 have a homogeneous structure(homogeneous alignment) where the liquid crystal molecules have apretilt angle of about 88.5°. As shown in FIG. 6( a), the average liquidcrystal director alignment 17 under AC voltage application is in thedirection of photoirradiation to the upper and lower substrates 12 whenthe substrates are viewed in plane. As shown in FIG. 6( b), directionsof absorption axes of the polarization plates (upper and lowerpolarization plates) arranged on the upper and lower substrates aredifferent from directions of photo-alignment treatment for the upper andlower substrates by 45° when the substrates are viewed in plane. Theliquid crystal display device of the present invention, produced throughthe above-mentioned alignment treatment for the alignment films andarrangement of the polarization plates, is a so-called VAECB (verticalalignment electrically controlled birefringence) device where thedirections of photoirradiation to the upper and lower substrates areopposite and parallel to each other and liquid crystal molecules arevertically aligned. In FIG. 6, the solid arrow shows a direction ofphoto-irradiation (a direction of photo-alignment treatment) to thelower substrate, and the dotted arrow shows a direction ofphoto-irradiation (a direction of photo-alignment treatment) to theupper substrate.

The case that the liquid crystal display device in accordance with thepresent Embodiment includes multi-domain pixels is mentioned below withreference to FIG. 9. In order to divide each pixel into four domains,the liquid crystal display device of the present Embodiment is subjectedto the following exposure step. Using a photomask 13 includinglight-shielding parts 14 each having a width that is a half width of onepixel (one sub-pixel), as shown in FIG. 7, a region corresponding to ahalf region of one pixel (one sub-pixel) is exposed in one direction (inFIG. 7, a direction from the front side to the back side of the paper)and simultaneously the rest half region is light-shielded by thelight-shielding part 14, first. Then, as shown in FIG. 8, the photomask13 is shifted by a half pitch of the pixel (one sub-pixel), and then,the region that has been exposed is light-shielded through thelight-shielding part 14 and a region which has not been light-shielded(an unexposed region in the step shown in FIG. 7) is exposed in thedirection opposite to the direction shown in FIG. 7 (in FIG. 8, adirection from the back side to the front side of the paper). As aresult, one pixel (one sub-pixel) is divided into two regions with thesame width, where liquid crystal molecules in one region and liquidcrystal molecules in the other region are pre-tilted in two mutuallyopposite directions. Such two regions are arrayed in a stripe pattern.

As mentioned above, each pixel (each sub-pixel) on each substrate isdivided into two domains by alignment division in such a way that thedomains are arranged at a regular pitch. Then, the both substrates arearranged (attached) so that the division directions (photo-alignmenttreatment directions) are perpendicular to each other between the upperand lower substrates 12, and then, liquid crystal materials free from achiral material are injected into the liquid crystal layer. As a result,as shown in FIG. 9( a), four domains where the alignment directions ofliquid crystal molecules that are positioned near the center in thethickness direction of the liquid crystal layer are different, morespecifically, substantially perpendicular to each other, among the fourdirections (FIG. 9( a), i to iv) can be formed. That is, the averageliquid crystal director alignment 17 under the AC voltage applicationbisects an angle made by the directions of photoirradiation to the upperand lower substrates 12 in each domain when the substrates are viewed inplane. The direction of photo-alignment treatment for the uppersubstrate (color filter substrate) (in FIG. 9( a), the direction shownby the dotted arrow) is the same as an absorption axis direction 16 of apolarization plate arranged on the upper substrate side, and thedirection of photo-alignment treatment for the lower substrate (drivingelement substrate) (in FIG. 9( a), the direction shown by the solidarrow) is the same as an absorption axis direction 15 of a polarizationplate arranged on the lower substrate side when the substrates areviewed in plane.

In each domain boundary, liquid crystal molecules on one of the pair ofsubstrates align in the same direction as an absorption axis directionof the polarizing plate, and liquid crystal molecules on the othersubstrate align almost perpendicular to the substrate. Accordingly, whenthe polarization plates are arranged in a Cross-Nicol state, the domainboundary does not transmit light even when a voltage is applied betweenthe substrates, and as a result, the boundary is observed as a darkline.

In this domain boundary, a photo-alignment film is redundantly exposed,generally. So in such a redundantly exposed-part (doubly-exposed part),a conventional photo-alignment film does not provide a uniform pretiltangle. In the doubly-exposed part of the conventional photo-alignmentfilm, the number of exposure treatment is asymmetry, and thereby ACimage sticking tends to be increased. However, use of the alignment filmof the present Embodiment effectively suppresses the AC image stickingin the doubly-exposed part and variation in the pretilt angle of liquidcrystal molecules in the doubly-exposure part.

As mentioned above, the liquid crystal display device of the presentEmbodiment can show excellent viewing angle characteristics, that is, awide viewing angle if having four domains where the alignment directionsof liquid crystal molecules are different from (substantiallyperpendicular to) one another.

The layout of the domains in the liquid crystal display device of thepresent Embodiment is not limited to that (division into four) shown inFIG. 9( a) and it may be a layout shown in FIG. 10( a). FIG. 10( a) is aplan view schematically showing: the average director alignment ofliquid crystal in one pixel (one sub-pixel); directions ofphoto-alignment treatment for a pair of substrates (upper and lowersubstrates); and a pattern of a multi-domain, in the case that theliquid crystal display device in Embodiment 1 has four domain differentfrom that in FIG. 9( a).

FIG. 10( b) is a schematic view showing directions of absorption axes ofpolarization plates in the liquid crystal display device shown in FIG.10( a). FIG. 10( c) is a schematic cross-sectional view taken along lineA-B in FIG. 10( a), and shows alignment directions of liquid crystalmolecules when an AC voltage not lower than a threshold voltage isapplied between the pair of substrates. In FIG. 10( a), the dotted arrowshows a direction of photoirradiation (a direction of photo-alignmenttreatment) to a lower substrate (driving element substrate); and thesolid line shows a direction of photoirradiation (a direction ofphoto-alignment treatment) to an upper substrate (color filtersubstrate).

In FIG. 10( c), the dotted line shows a domain boundary.

The domains of this Embodiment are formed in the following procedures.As shown in FIG. 10( a), each pixel (each sub-pixel) on each of a pairof substrates is divided into two domains by alignment division in sucha way that the domains are arranged at a regular pitch. As shown in FIG.10( a), the both substrates are arranged (attached to each other) insuch a way that when the substrates are viewed in plane, the divisionaldirections (photo-alignment treatment directions) are perpendicular toeach other between the upper and lower substrates 12 and that the uppersubstrate is shifted by a quarter of a pixel pitch in the directionshown by the dotted arrow in FIG. 10( a). As a result, as shown in FIG.10( a), four domains (in FIG. 10( a), i to iv) where the alignmentdirections of liquid crystal molecules that are positioned near thecenter in the thickness direction of the liquid crystal layer aredifferent depending on the domain, more specifically, substantiallyperpendicular to each other can be formed. That is, as shown in FIG. 10(a) the average liquid crystal director alignment 17 under the AC voltageapplication bisects an angle made by the directions of photoirradiationto the upper and lower substrates 12. As shown in FIG. 10( b), in thisEmbodiment, when the substrates are viewed in plane, the direction ofphoto-alignment treatment (in FIG. 10, the direction shown by the solidarrow) for the upper substrate (color filter substrate) is the same asthe absorption axis direction 16 of the polarization plate arranged onthe upper substrate side, and the direction of photo-alignment treatment(in FIG. 10, the direction shown by the dotted arrow) for the lowersubstrate (driving element substrate) is the same as the absorption axisdirection 15 of the polarization plate arranged on the lower substrateside. When no voltage is applied between the upper and lower substrates,the liquid crystal molecules align substantially vertically to the upperand lower substrates attributed to alignment regulating force of thealignment film. When a voltage not lower than a threshold voltage isapplied between the upper and lower substrates, the liquid crystalmolecules 11 twist almost 90° between the upper and lower substrates andalign in directions different depending on the four domains.

4. Production Method of Liquid Crystal Display Device

The production method of the liquid crystal display device in accordancewith the present Embodiment is mentioned below.

First, a pair of substrates on which an alignment film is not arrangedis produced by a common method.

A driving element substrate is produced as one of the pair of substratesas follows: (1) scanning signal lines, (2) driving elements, such as aTFT, (3) data signal lines, and (4) pixel electrodes, which aretransparent electrodes, are successively formed on a glass substrate.The scanning signal lines and the data signal lines are arranged in amatrix pattern to intersect with each other on the substrate. Aninsulating film is arranged between the scanning signal lines and thedata signal lines. The driving element and the pixel electrode arearranged at each intersection of the scanning signal lines with the datasignal lines. Components in the driving element substrate may be formedfrom a material commonly used.

A color filter substrate (CF substrate) is produced as the othersubstrate of the pair of substrates as follows: (1) a black matrix (BM),(2) color filters, (3) a protective film, and (4) common electrodes,which are transparent electrodes, are successively formed on a glasssubstrate. The BM is arranged in a lattice pattern, and the color filteris arranged in each region surrounded by the BM. Components in the CFsubstrate may be formed from materials commonly used.

Then an alignment film-forming step is performed. This is mentioned indetail above in 1. the alignment film material and 2. preparation methodof alignment film. So a specific example thereof is mentioned below. Avarnish is prepared from the copolymer of an imide derivative, an amidederivative, and the like, containing the above-mentioned photofunctionalgroup with an imide derivative, an amide derivative, and the like,containing no photofunctional group. After being printed on a substrate,the varnish is pre-baked on a hot plate for pre-baking at 90° C. for 1minute and then post-baked on a hot plate for post-baking at 200° C. for60 minutes. The varnish is printed on the substrate to give an alignmentfilm with a thickness of 100 nm after the baking. After being cooled toroom temperatures, the substrate is irradiated with P-polarized UV lightwith a polarization degree of 10:1 at an exposure energy of 20 mJ/cm²from a direction making an angle of 40° with respect to the normaldirection of the substrate surface. In such a manner, the alignment filmin accordance with the present Embodiment is formed. The alignmenttreatment for the alignment film (the alignment film-exposing step) maybe performed after the below-mentioned spacer-arranging step.

Then the spacer-arranging step is performed. A cell gap-controllingmember such as plastic beads (product of SEKISUI FINE CHEMICAL CO.,LTD., trade name: Micro-pearl, 3.5 μm in diameter) in a desired amount(density: four to five beads per 100 μm²) is dry-sprayed. The spacer maybe arranged by ink-jet printing, i.e., by printing an ink containing thecell gap controlling member (adhesive beads) at a desired position. Ifnecessary, the substrate may be heated at a predetermined temperature(for example, about 100 to 200° C.) in order to sufficiently fix theadhesive beads to the substrate. In addition, the spacer may be arrangedby forming a photo spacer at a predetermined position using aphotosensitive resin material before forming the alignment film.

Then a sealing member-arranging step is performed. A sealing member isapplied to the substrate including no spacers. The sealing member ispreferably applied by screen printing or with a dispenser. STRUCT BONDXN-21S, a product of Mitsui Chemicals, Inc., a photo thermal sealingagent, product of Kyoritsu Chemical & co., ltd., and the like, may bepreferably used as the sealing member, for example.

Then a liquid crystal material-injecting step is performed. Vacuuminjection or a drop-and-fill process is preferably used for injectingthe liquid crystal material. If the vacuum injection is employed, aphotocuring bond produced by ThreeBond Co., Ltd. or SEKISUI CHEMICALCO., LTD., is preferable as the sealing agent.

Like a conventional method, a polarization plate-attaching step, and amodule-producing step are performed, and as a result, the liquid crystaldisplay device of the present Embodiment can be completed.

5. AC Image Sticking Evaluation Test

A mono-domain VATN liquid crystal display device shown in FIG. 5 wasproduced and subjected to an AC image sticking evaluation test. Resultsof the test are mentioned below. The characteristics of AC imagesticking (AC characteristics) were evaluated based on characteristicsdepending on an introduction ratio of an imide derivative, an amidederivative, and the like, containing a vertical alignment functionalgroup and not having a photofunctional group relative to an imidederivative, an amide derivative, and the like, containing aphotofunctional group. The liquid crystal device included an ITOelectrode-including substrate where an ITO transparent electrode wasdivided into two (electrodes 18 a and 18 b) as shown in FIG. 11.

Polymers (polyimides or polyamides) with introduction ratios (% byweight) of 0%, 8%, 15%, 25%, 40%, and 50%, respectively, were prepared,the ratios being a vertical alignment diamine unit not containing aphotofunctional group relative to a photo-alignment diamine unit. Amonomer of the photo-alignment diamine unit was selected from thephotofunctional group-containing compounds represented by the aboveformulae (1) to (4). A monomer of the vertical alignment diamine unitwas selected from the compounds represented by the above-mentionedformulae (10) to (19). An acid anhydride was selected from the compoundsrepresented by the above-mentioned formulae (20) to (26).

Then a varnish was prepared from the above-mentioned solvent forprinting and applied on the ITO electrode-including substrate through aprinting step by ink-jet printing or with a drum roll-coating. Then theITO electrode-including substrate to which the varnish had been appliedwas pre-baked at 90° C. for one minute with a hot plate for pre-baking.After the pre-baking, the alignment film had a thickness of about 100nm. Then the substrate was post-baked at 200° C. for 40 minutes with ahot plate for post-baking.

Then an ink containing the cell gap controlling member (dispersionliquid) was applied to a predetermined position (a light-shielding partin a non-display region) by ink-jet printing and dried at a roomtemperature of 24° C. to arrange the cell gap controlling member on theITO electrode-including substrate.

After being cooled to room temperature, the ITO electrode-includingsubstrate including the cell gap controlling member was provided with aphoto-alignment treatment by being irradiated with (exposed to) UVlight. More specifically, the substrate was irradiated with P-polarizedUV light with a polarization degree of 10:1 at an exposure energy of 20mJ/cm² from a direction making an angle of 40° with respect to thenormal direction of the substrate surface.

Then the sealing member was printed on the other substrate, and the bothsubstrates were attached to each other. The cell gap between thesubstrates was 3.5 μm. Then under heating at 60° C., liquid crystalmaterials with negative dielectric anisotropy (product of Merck KGaA,MLC6610, Δn: 0.09, Δ∈: −2.4, Tni: 90° C.) were injected between thesubstrates, which had undergone the above-mentioned steps, and sealed.Then as a realignment treatment step, the liquid crystal display devicewas heated by being kept in an oven at 130° C. for 30 minutes, and thenthe device was rapidly cooled to a room temperature of 24° C. at a rateof about 4° C./min. The average pretilt angle of the liquid crystallayer was about 88.5° to 89.5°. In the liquid crystal display device,two polarization plates (an upper polarization plate 23 a and a lowerpolarization plate 23 b) were attached in such a way that theirabsorption axes are in a cross-Nicol state.

The evaluation method of AC characteristics is mentioned.

The first photographing was performed as follows.

In an initial state (before application of an AC voltage (30 Hz, 7V)),an AC voltage (30 Hz) was applied to the electrodes 18 a and 18 b of theliquid crystal display device prepared as mentioned above. Withincreasing the voltage by 0.05V from 2.15V to 2.5V, images (displaystate) which were displayed by a liquid crystal display device 19 ateach voltage were taken with a digital camera (product of Canon Inc.,trade name: Eos Kiss Digital N) 24 that was located 40 cm away. In thisevaluation test, the AC voltage was applied to the electrodes with asignal generator (product of Iwatsu Electric Co., Ltd., SG-4115).

The second photographing was performed as follows.

An AC voltage (30 Hz, 7V) kept being applied to only the electrode 18 bfor a certain period of time (x) as shown in FIG. 12, and then an ACvoltage (30 Hz) was applied to the electrodes 18 a and 18 b. Similarlyto the first photographing, with increasing the voltage by 0.05V from2.15V to 2.5V, images at each voltage were taken. The time x was varied,and the second photographing was repeatedly performed. The voltage of2.15V to 2.5V (which was applied by 0.05V) was applied only during thesecond photographing, just for about 1 minute. Accordingly, this timewas much shorter than the time x. The taken images were analyzed in thefollowing procedures (I) to (III). An image-processing software (productof Media Cybernetics, Inc., Image-Pro Plus) was used for the analysis.

(I) With respect to the images that were taken in the secondphotographing after 40 hours' voltage application, i.e., when x=40,ratios between a luminance of the electrode 18 b and a luminance of theelectrode 18 a (a luminance of the electrode 18 b)÷(a luminance of theelectrode 18 a) when the respective voltages were applied (by 0.05V from2.15V to 2.5V) were calculated. Then a voltage at which the ratio showsthe maximum value, that is, a voltage corresponding to the maximumluminance ratio (ΔT) was determined.(II) With respect to images that were taken when the voltagecorresponding to the maximum luminance ratio, determined in (I), wasapplied among the images taken in the first and second photographing, aluminance ratio (a luminance of the electrode 18 b)÷(a luminance of theelectrode 18 a) was calculated, and the maximum luminance ratio (ΔT) ateach AC voltage (30 Hz, 7V) holding time, i.e., at each time x, wasdetermined.(III) The time (x) during the AC voltage (30 Hz, 7V) application wasplotted along the x-axis and the maximum luminance ratio (ΔT) at eachtime x, determined in the above (II), was plotted along the y-axis.

Before and after the AC voltage (30 Hz, 7V), a DC off-set value when avoltage of 2.3V to 2.4 V was applied was determined to be almost 0.Accordingly, this ΔT evaluation shows that the image sticking was causedby only the influences in AC mode, not in DC mode.

The evaluation results of the AC characteristics are mentioned below.FIG. 15 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) including an alignment film with anintroduction ratio (% by weight) of a vertical alignment diamine unitnot containing a photofunctional group to a photo-alignment diamine unitof 0%. Similarly to FIG. 14, FIGS. 16 to 20 are graphs showing ΔTcharacteristics at introduction ratios of 8% (FIG. 16), 15% (FIG. 17),25% (FIG. 18), 40% (FIG. 19), and 50% (FIG. 20), respectively. In FIGS.15 to 20, a plurality of graphs are plotted and show ΔT characteristicsof a plurality of evaluation cells that are produced using the samematerials under the same conductions. FIG. 21 shows a graph showing ΔTcharacteristics after 40 hour's AC voltage application to the evaluationcells with introduction ratios of 0%, 15%, 25%, 40%, and 50%. In FIG.21, the rhombic marker (♦) shows the average value and the range (I)partitioned by the line segment shows a range from the largest value andthe smallest value.

This evaluation results show that the ΔT characteristics are moreimproved and the AC image sticking is more suppressed as theintroduction ratio of the vertical alignment diamine unit not containinga photofunctional group to the photo-alignment diamine unit isincreased. Although not shown in drawings, it was determined that the ΔTwas about 1.08 when the introduction ratio was 4%. Thus, it was shownthat when the introduction ratio was 4% to 40%, the ΔT was reduced to1.08 or less and the AC image sticking could be more effectivelysuppressed. It was also shown that when the introduction ratio was 15%to 40%, the ΔT was reduced to 1.06 or less and the AC image stickingcould be particularly effectively suppressed. The average pretilt angleof the liquid crystal layer was 88.3° in the liquid crystal layer whenthe introduction ratio was 0%; the angle was 88.5° at 4%; the angle was88.8° at 8%; the angle was 89.2° at 15%; and the angle was 89.5° at 40%.Thus, the higher the introduction ratio is, the larger the averagepretilt angle of the liquid crystal layer tends to be.

The present application claims priority to Patent Application No.2007-80289 filed in Japan on Mar. 26, 2007 under the Paris Conventionand provisions of national law in a designated State, the entirecontents of which are hereby incorporated by reference.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a basic structure of the polymer included in the alignmentfilm material in accordance with Embodiment 1.

In FIG. 1, the part encircled by the solid line is a unit derived froman acid anhydride (acid anhydride unit); the part encircled by thedashed line is a unit derived from a diamine having a side chaincontaining a photofunctional group (photo-alignment diamine unit); andthe part encircled by the dashed-dotted line is a unit derived from adiamine having a side chain containing a vertical alignment functionalgroup (vertical alignment diamine unit).

FIG. 2 is a cross-sectional view schematically showing a vicinity of thealignment film surface in the liquid crystal display device inaccordance with Embodiment 1 and explaining a mechanism for suppressingAC image sticking due to side-chain deformation.

FIG. 2( a) shows an initial state.

FIG. 2( b) shows a state where an electrical field is applied to theliquid crystal layer.

FIG. 2( c) shows a state where the application of the electrical fieldto the liquid crystal layer is stopped.

FIG. 3 is a cross-sectional view schematically showing a vicinity of thealignment film surface in the liquid crystal display device inaccordance with Embodiment 1 and explaining a mechanism for suppressingAC image sticking cause by the liquid crystal adsorption.

FIG. 3( a) shows an initial state.

FIG. 3( b) shows a state where an electrical field is applied to theliquid crystal layer.

FIG. 3( c) shows a state where the application of the electrical fieldto the liquid crystal layer is stopped.

FIG. 4 is a perspective view schematically showing a relationshipbetween a photo-alignment treatment direction and a pretilt direction ofa liquid crystal molecule in accordance with Embodiment 1.

FIG. 5( a) is a plan view schematically showing a director alignment ofliquid crystal in one pixel (one sub-pixel); and directions ofphoto-alignment treatment for a pair of substrates (upper and lowersubstrates) in the case that the liquid crystal display device inEmbodiment 1 is a mono-domain device.

FIG. 5( b) is a schematic view showing directions of absorption axes ofpolarization plates arranged in the liquid crystal display device shownin FIG. 5( a).

FIG. 5 (a) shows a state where the photo-alignment treatment directionsare perpendicular to each other between a pair of substrates and an ACvoltage not lower than a threshold voltage is applied between the pairof substrates.

In FIG. 5( a), the solid arrow shows a direction of photo-irradiation (adirection of photo-alignment treatment) for a lower substrate; and thedotted arrow shows a direction of photo-irradiation (a direction ofphoto-alignment treatment) for an upper substrate.

FIG. 6( a) is a plan view schematically showing a director alignment ofliquid crystal in one pixel (one sub-pixel); and directions ofphoto-alignment treatment for a pair of substrates (upper and lowersubstrates) in the case that the liquid crystal display device inEmbodiment 1 is a mono-domain device.

FIG. 6( b) is a schematic view showing directions of absorption axes ofpolarization plates arranged in the liquid crystal display device shownin FIG. 6( a).

FIG. 6( a) shows a state where photo-alignment treatment directions areparallel and opposite to each other between the pair of substrates andan AC voltage not lower than a threshold voltage is applied between thepair of substrates.

In FIG. 6( a), the solid arrow shows a direction of photo-irradiation (adirection of photo-alignment treatment) for a lower substrate; and thedotted arrow shows a direction of photo-irradiation (a direction ofphoto-alignment treatment) for an upper substrate.

FIG. 7 is a cross-sectional view schematically showing a firstarrangement relationship between substrates and a photomask in aphoto-alignment treatment process in accordance with Embodiment 1 wherealignment division is performed by proximity exposure using an alignmentmask.

FIG. 8 is a cross-sectional view schematically showing a secondarrangement relationship between substrates and a photomask in aphoto-alignment treatment process in accordance with Embodiment 1 wherealignment division is performed by proximity exposure using an alignmentmask.

FIG. 9 (a) is a plan view schematically showing: the average directoralignment of liquid crystal in one pixel (one sub-pixel); directions ofphoto-alignment treatment for a pair of substrates (upper and lowersubstrates); and a arrangement pattern of a multi-domain, in the casethat the liquid crystal display device in Embodiment 1 is a four-domaindevice.

FIG. 9( b) is a schematic view showing directions of absorption axes ofpolarization plates in the liquid crystal display device shown in FIG.9( a).

FIG. 9( a) shows a state where an AC voltage not lower than a thresholdvoltage is applied between the pair of substrates.

In FIG. 9( a), the solid arrow shows a direction of photoirradiation (adirection of photo-alignment treatment) to a lower substrate (drivingelement substrate); and the dotted arrow shows a direction ofphotoirradiation (a direction of photo-alignment treatment) to an uppersubstrate (color filter substrate).

FIG. 10( a) is a plan view schematically showing: the average directoralignment of liquid crystal in one pixel (one sub-pixel); directions ofphoto-alignment treatment for a pair of substrates (upper and lowersubstrates); and a pattern of a multi-domain, in the case that theliquid crystal display device in Embodiment 1 has four domain differentfrom that in FIG. 9( a).

FIG. 10( b) is a schematic view showing directions of absorption axes ofpolarization plates in the liquid crystal display device shown in FIG.10( a).

FIG. 10( c) is a schematic cross-sectional view taken along line A-B inFIG. 10( a), and shows alignment directions of liquid crystal moleculeswhen an AC voltage not lower than a threshold voltage is applied betweenthe pair of substrates.

In FIG. 10( a), the dotted arrow shows a direction of photoirradiation(photo-alignment treatment direction) to a lower substrate (drivingelement substrate); and the solid arrow shows a direction ofphotoirradiation (photo-alignment treatment direction) to an uppersubstrate (color filter substrate).

In FIG. 10( c), the dotted line shows a domain boundary.

FIG. 11 is a plan view schematically showing transparent electrodes inan evaluation cell (liquid crystal display device) used in the AC imagesticking evaluation test.

FIG. 12 is a plan view schematically showing a display state of theevaluation cell (liquid crystal display device) when a current isapplied to the cell in the AC image sticking evaluation test.

FIG. 13 is a plan view schematically showing a display state of theevaluation cell (liquid crystal display device) when the AC imagesticking evaluation test is performed.

FIG. 14 is a side view schematically showing an arrangement relationshipbetween the evaluation cell (liquid crystal display device) and adigital camera when the AC image sticking evaluation test is performed.

FIG. 15 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 0% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 16 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 8% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 17 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 15% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 18 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 25% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 19 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 40% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 20 is a graph showing ΔT characteristics of an evaluation cell(liquid crystal display device) having an alignment film with 50% byweight of an introduction ratio of the vertical alignment diamine unitcontaining no photofunctional groups to the photo-alignment diamineunit.

FIG. 21 is a graph showing ΔT characteristics after 40 hour's AC voltageapplication to the evaluation cells with introduction ratios of 0%, 15%,25%, 40%, and 50%.

In FIG. 21, the rhombic marker (♦) shows the average value and the range(I) partitioned by the line segment shows a range from the largest valueand the smallest value.

FIG. 22 is a cross-sectional view schematically showing a vicinity of aconventional photo-alignment film surface and explaining a mechanism forgeneration of AC image sticking due to side-chain deformation.

FIG. 22( a) shows an initial state.

FIG. 22( b) shows a state when an electrical field is applied to aliquid crystal layer.

FIG. 22( c) shows a state when the application of the electrical fieldto the liquid crystal layer is stopped.

FIG. 23 is a cross-sectional view schematically showing a vicinity of aconventional photo-alignment film surface and explaining a mechanism forgeneration of AC image sticking due to liquid crystal adsorption.

FIG. 23( a) shows an initial state.

FIG. 23( b) shows a state when an electrical field is applied to aliquid crystal layer.

FIG. 23( c) shows a state when the application of the electrical fieldto the liquid crystal layer is stopped.

EXPLANATION OF NUMERALS AND SYMBOLS

-   10: Alignment film-   11, 111: Liquid crystal molecule-   12: Upper and lower substrates-   13: Photomask-   14: Light-shielding part-   15: Direction of absorption axis of polarization plate on lower    substrate side-   16: Direction of absorption axis of polarization plate on upper    substrate side-   17: Average liquid crystal director direction under AC voltage    application-   18 a, 18 b: Electrode-   19: Liquid crystal display device-   20, 120: Liquid crystal layer-   21: Side chain containing a photofunctional group-   22: Side chain containing a vertical alignment functional group-   23 a: Upper polarization plate-   23 b: Lower polarization plate-   24: Digital camera-   25, 125: Main chain-   130: Photo-alignment film-   131: Side chain

The invention claimed is:
 1. A liquid crystal display device comprising:a pair of substrates; a liquid crystal layer comprising liquid crystalmolecules; and an alignment film, the liquid crystal layer beinginterposed between the pair of substrates, the alignment film beingarranged on a liquid crystal layer side-surface of at least one of thepair of substrates, wherein the alignment film is obtainable byproviding a film with an alignment treatment by photoirradiation, thealignment film being formed from an alignment film material, thealignment film material including a copolymer containing a firstconstitutional unit and a second constitutional unit, wherein astructure of the first constitutional unit is different than a structureof the second constitutional unit before and after the photoirradiation;the first constitutional unit exhibiting a property of controllingalignment of the liquid crystal molecules by photoirradiation, and thesecond constitutional unit exhibiting the property of controllingalignment of the liquid crystal molecules regardless ofphotoirradiation.
 2. The liquid crystal display device according toclaim 1, wherein the alignment film is a vertical alignment film thataligns the liquid crystal molecules vertically, and aligns the liquidcrystal molecules in such a way than a average pretilt angle of theliquid crystal layer is 87° or more, and 89.5° or less, and the liquidcrystal molecules are uniformly aligned in a plane of the alignmentfilm.
 3. The liquid crystal display device according to claim 1, whereinthe first constitutional unit has a side chain containing a cinnamategroup as a photofunctional group, and the second constitutional has aside cabin containing a steroid skeleton.
 4. The liquid crystal displaydevice according to claim 1, wherein the polymer has at least one mainchain structure selected from the group consisting of a polyamic acid, apolyimide and a polysiloxane.
 5. The liquid crystal display deviceaccording to claim 1, wherein a ratio by weight of a monomer componentof the second constitutional unit to a monomer component of the firstconstitutional unit is 4% or more.
 6. The liquid crystal display deviceaccording to claim 1, comprising pixels arranged in a matrix pattern,the pixels including a pixel electrode and a common electrode, the pixelelectrode being arranged in a matrix pattern on a liquid crystal layerside-surface of one of the pair of substrates, the common electrodebeing arranged on a liquid crystal layer side-surface of the othersubstrate, wherein the pixels include two or more domains adjacent toeach other, average directors in the four domains have a 4-foldrotational symmetry, and directions of the average directors change insuch a manner that the average directors rotate at an interval of 90degrees.